How accurate is your TPMS?

I have seen a number of people express some concern about the accuracy of the pressure readings from their TPMS. You can read my previous responses HERE  and HERE

I have also pointed out that the primary purpose or "job" of a TPMS is to warn the driver of a pressure LOSS.

I am inclined to think that some are still concerned with their exact pressure reading. I also have to wonder how some people are comparing various pressure readings reported by their TPMS. If they have external sensors, the simple act of removing and replacing a sensor can allow some air to escape. While I doubt that the small loss of some air will result in a meaningful pressure differential it does add some uncertainty.

So the engineering DNA in me kicked in and I a devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from Tire Traker and one set of 6 internal sensors is from Truck Systems Technology.

The question is: How do I make the test both fair and useful. I decided to eliminate as many variables as possible and get all 12 readings from the same air chamber at the same time and compare them all against my personal digital hand gauges that I have checked against an ISO certified laboratory gauge.
Note my hand gauge reads to 0.5 psi which is way more precise than anyone needs for checking tires in normal highway use.

Here is the test fixture I made.

It has 6 bolt in valves for mounting the external sensors plus a valve for me to use with my hand gauge. Also since safety is always of concern when dealing with a pressure vessel I added a pop-off valve.  One end of the fixture has a cap that can be removed so I can place the internal TPM sensors inside the 4" tube. It also has a regulated air supply to compensate for the very slow leak around the cap and an analog dial gauge that allows me to constantly, visually monitor the air pressure to ensure the use of my hand gauge does not result in a pressure change that isn't immediately compensated for by the regulated air supply.

Here are the results of my comparison test. The target pressure is 80.0 psi as reported by my handheld digital gauge.

Set A  1 reading of 78 psi,  5 readings of 79 psi
Set B  2 readings of 78, 2 readings of 79 and 2 readings of 80 psi

I also recorded the temperature.
Set A  4 readings of 66 F, one each of 64 and 68F
Set B  4 readings of 69 F  and 2 readings of 68

I do not consider any of the differences in the readings of pressure or temperature to be significant or meaningful for a TPMS.  I would consider the pressure readings from all 12 sensors to essentially be equivalent.
You can learn a bit more about what a "meaningful" measurement is HERE.

Why isn't my TPMS accurate?

Comment on an RV forum
"I just bought a new TST 507  TPMS (tire pressure monitoring system) and  I gotta say they could improve the documentation.
Definitely confusing as setting up is a bit complex. At one point you are told to press a button and wait for the second beep (not the first!) before setting the pressures or whatever.
But once that's over, it works as advertised. It does seem to take a while, once turned on, for all the sensor readings to be displayed. After that it just cycles through them every 5 s, and beeps when one is over or under.
Not sure how accurate the psi readings are. Will have to get out my pressure gauge and compare."
+++++++++++++++=
My reply

TPMS are primarily designed to WARN of a loss in pressure and are not intended to be your primary source of accurate pressure measurement. In my experience ALL TPMS will have one or more sensors reading a psi or two different than a hand-held digital gauge. For that matter even hand-held gauges are seldom accurate to +/- 1.0 PSI based on the calibration checks I offer at my tire seminars.

IMO best practice is to set your tire pressure as close as possible to the desired CIP (cold inflation pressure) using your personal, master digital hand gauge and then install the TPMS sensors. Wait about 5 min for pressures and readings to stabilize, then do a pressure check on the monitor. You will note slight variation, but all should be within the specified range (some spec +/- 2%  some spec +/- 2 psi) You will find that after a few days use you will get comfortable with the pressure readings showing some variation from each other. With a little experience you will get comfortable and know the range of normal variation.

What I do...
Each morning when I get up, I first turn on the TPMS monitor and go get my coffee. By the time I am done with the 1st cup - about 10 min - all the sensors have sent in a reading. I step through each tire on the system and based on experience, I know if the tires are ready to go as far as being properly inflated. Yes, the numbers vary, but as long as they all are in a range of a couple psi from what I got when I last set the tire pressure all is good.

With the above practice I am getting the "cool tire" pressure reading which is the only reading of importance. With my +10% over the minimum inflation needed for my Motorhome, I don't have to worry about a daily +/- 2 to 4 psi variation. However, if I have a few days in a row with the display pressures consistently showing a few psi lower than my CIP goal I know it's time to plan on a stop at the truck stop for a shot of high pressure air.

##RVT789

Basic tire inflation procedure - Motorhomes

Thought a short post covering the important points of setting and ensuring you have the necessary inflation on your motorhome tires was in order.

 NOTE Trailers have slightly different needs for establishing the "set" pressure than motorhomes but the rest of this post would apply to towables too.

1. Ensure your gauge is sufficiently accurate. To me this means +/- 3 psi of the real pressure. This level of accuracy is easy to confirm yourself if you follow these steps:
      A. Get and keep a "Master" gauge. This is not used for checking the air in your tires but is used to confirm that your daily use gauge is sufficiently accurate and to allow you to adjust the reading of your daily gauge accordingly. I have provided an example of what a low cost "Master" gauge could be in THIS post. The master gauge should NOT be kept in your tool box but in a box or case inside the RV where it is protected from damage.
     B. At least once a month, measure the air in one of your tire with the Master gauge and immediately get a reading with your daily gauge. You now know how much to adjust your reading when setting your tire pressures. If your daily gauge reads 3 psi lower than the Master then you need to set your tires 3 psi high when using your daily gauge.

2. Learn the actual load on each position of your RV. This has been covered in a number of earlier posts that have the label "Load". Please review those posts if you do not know how to learn your "corner" loading. The minimum inflation for all tires on any one axle is based on the inflation needed for the heaviest loaded end of the axle. ALL tires on an axle should have the same cold inflation.

3. Establish your "Set" or "Goal" pressure pressure that you should have each morning of a travel day. I suggest this be 10% above the minimum needed to carry the load on your tires you learned in step 2. above

4. Set your TPMS to provide warning if you have lost air to the point of being 10% below the Set Pressure. This should provide adequate warning to allow you to safely get off the road or at least out of traffic. If you don't already have a TPMS then get one. If you don't think you need a TPMS then I would challenge you to try this experiment. Get some black tape and cover all your dash gauges and see how comfortable you are traveling down the highway without having any of the factory provided warning instruments you have become accustomed to.

I feel these steps are the minimum every motorhome owner should take



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New Year Resolution on Tires

Happy New Year!

Well it's that time when we make Resolutions. Many make ones that they know, deep down they will probably not keep. You know the ones like "Go a diet and loose 25# or to start walking 5 miles every day.

Well I would like to offer some suggestions for resolutions that won't be too difficult to keep and could pay BIG dividends in improved fuel mileage or decreased chance of a tire failure or improved tire wear.


#1. Get a your own "Master" tire gauge. This should be a digital gauge that you keep packed away and not in the bottom of your tool box. You can use this gauge to check your regular hand gauge against once every few weeks.
My personal favorite is this Accutire gauge
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You don't need a $100 special truck gauge as you simply use this gauge to measure a front tire and compare the reading with the reading on your normal gauge. I have found this gauge to be accurate to +/- 1/2 psi or better, when checked against ISO Laboratory test instruments,.which is more than accurate enough. If your regular gauge is within a couple psi of this one that should be good enough PLUS you know if your regular gauge is reading high or low.

#2. On your first trip pull off the Interstate at a truck stop that has scales and confirm the current load on each axle. Now hopefully you have had each tire (for trailers) or each "corner" for motorhomes weighed in the past. Doing a quick axle weight check will confirm you haven't made a significant change in your loading as the axle total should match the total of the individual tires you previously had measured.

#3 At least once each year have your tires completely inspected by a tire dealer that sells your brand of tire. If you have a trailer you should have a "Free Spin" inspection to check for Radial or Lateral runout as seen in this video.  I covered the examination of this "Failed" tire in my post on how to inspect your tires.

#4 And now the tough one here. Go to my blog Archive (lower right side) and go back to oldest post and "Resolve" to review four posts each week. Now you don't have to remember everything but I do hope that when you are done you will have an idea of where to go when a question comes up and you will hopefully remember reading something on the topic of interest and you can then go back and re-read the specific post.

May you all have a Safe and Tire Problem Free 2015.
Roger Marble   aka Tireman9


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Quick Summary here

I have been following a number of RV forums and want to do a bit of a summary for those that don't have the time or interest to read a number of posts on this Blog: Remember more than half of the thousands of RVs that have been checked have been found to have one or more tire and/or axle in overload. So nect time you camp you can be pretty sure that one or both of the RVs parked next to you are overloading a component.

- You should have a Digital tire gauge. I have posted the results of my tests of dozens of gauges that RVrs were using and the bottom line is that about 15% were off by more than 5 psi which  IMO makes that tire gauge a good door stop or tent peg. I have provided information on how to do your own gauge "Calibration Check" to confirm the accuracy of the gauge you use daily and how to maintain your "Master Gauge"

- If you follow the advice from the two Tire Engineers on this thread you will be setting the pressure based on the appropriate method here:
     - On your TV inflate to pressure based on the Door sticker and Owner's manual for "Fully Loaded" operation. BUT get on a scale at least once to confirm you are not overloaded.
     - If you have a multi-Axle trailer you should be running the pressure on the tire sidewall. I have written in my Blog and provided the technical justification and need to follow this advice. You should still go across a scale and calculate the actual tire loads to confirm you are not overloading one or more tires
      - If you have Motorhome you need to confirm your unit's "corner weights". Use one of the worksheets I have provided links to. Use Load/Inflation chart from your tire manufacturer, to confirm you are not overloaded and identify the heavier loaded tire on each axle. Using the pressure for that load add 10% (without exceeding the wheel max) and that is your Cold set pressure for all tires on that axle.
      - If you follow the above there should be no reason to be chasing your tire inflation around unless you have a major change in Morning temperature ( More than say 30°F ) as you would only need to "Top Off" the air once a month at most.

- Don't be too anal about inflation. Variations of 1 to 3 psi day to day are normal. You should not need to make adjustments unless the Cold pressure is 4 or 5 psi away from your goal. (Multi-axle trailers might try and get no lower than 3 psi below tire sidewall pressure)

- TPMS are a WARNING device and like Insurance we should all be using them. Also just like Insurance they cannot "Prevent" bad stuff from happening but with luck they may give sufficient advance warning to lessen the impact of the bad stuff that does happen.

- If you are parked for more than a day you need to cover your tires that will be in direct sunlight with WHITE tire covers. This protects against both UV and higher temperature damage which can shorten tire life.

Measurable vs Meaningful

It is important to understand the difference between these two terms. Sometimes people assume that if something can be measured it is important.
Over the years as technology has advanced it becomes more and more easy to detect ever smaller quantities of material or forces.  As an engineer I too often hear people expressing concern over some new discovery of x parts per Million or even x parts per Billion of some contaminant to air or water, or foreign material in some product. Some of these numbers are important and meaningful while others are not really significant so there should not be a cause for concern. The challenge is to know when some measurement is meaningful.

Some of you are asking yourself what this has to do with RV tires. Well if you have read many of my posts here in this blog or on various RV Forums you no doubt have seem me and others saying you need to know the actual load on each tire and not just guess. Also we tell you how important it is to have the proper inflation in your tires.

What made me think of a need for this topic is the occasional complaint I get from some that think we are suggesting a need to get their RV weighed every time they travel or add an item to their packing list.. They basically say they are not going to do that so ask "Why bother to weigh the RV at all?"

Lets step back and look at tire Load & Inflation tables. For highway use tires the tables have 5 psi increments and depending on the size tire that 5 psi increment can provide as little as 110# additional load carrying capacity for a small ST type tire to 320# or more additional capacity to a large Class-A tire. If we do some simple math that means on a single axle small Trailer you would need to adjust your inflation if you were to increase the load in the trailer by 220# but the Class-A RV with rear duals and a tag you might be able to tolerate (8x320#) or 2,560# additional load (assuming perfect load distribution).

If your hobby was going to yard sales and picking up cast iron lawn ornaments it might be possible for a couple good sized items at (115# each) to be meaningful so you would need to increase your tire inflation on your small teardrop trailer (assuming you were not already running the tire max). The owner of the 40' Class-A on the other hand could probably pick-up a couple dozen items that each weighed 105# and if the load was perfectly distributed still not be overloading the tires.
Now clearly we can count (measure) the difference in the additional load of the two 115# items and the load of twenty four 105# items. In both cases the additional load is easily measured but while 120# could be very meaningful to the small trailer owner an increase in load of 120# would certainly NOT be meaningful to the Class-A RV owner. Maybe you now have a better understanding of the difference between measurable and meaningful.

In a previous post, I provided information of the accuracy of pressure gauges, showing that I find that 10% to 15% of the gauges I have checked are off by 5 psi or more. While my personal gauges are certified accurate to +/- 0.5psi, that is way more accurate you you "need" if you are setting your cold inflation to give yourself a 10psi Safety Margin over what is needed for your normal full load. If however you are running loads that fall just below the load indicated for the cold inflation you are using, then BOTH your gauge needs to be more accurate than +/-5 psi and your scale reading needs to be accurate to less than half of the load increment in the table.

Example: Tables for a 255/80R22.5 show 4,805# @ 95 and 4,975@ 100 psi so your gauge needs to be at least +/- 2.5 psi from actual certified measurement and your load needs to be measured to an accuracy of at least +/- 85# if you are using 100 psi as your cold inflation.

Tire Inflation accuracy

I recently read a number of posts on some RV forums on tire gauge, TPMS pressure accuracy and proper inflation levels. I will try to answer a number of questions in this post.

At a number of RV events I have tested different gauges against a digital 0.5 psi reading gauge that has been checked against QS9000 ISO/IEC 17025 approved master laboratory pressure gauge and found accurate to +/- 0.5psi. We have found that about 15% of the tested gauges were wrong by more than 5psi with a few off by more than 12psi. In general the most accurate were digital. The least accurate were the "stick" gauges with the sliding scale.

I have suggested that if you are accurate to within 5% of your goal cold tire inflation you are probably OK but obviously more accurate is better. The 5% tolerance is based on an assumption that you have added 10% to the minimum required per measurement of actual tire load and consultation with your tire companies Load/Inflation tables. It has also been suggested that using any gauge is better than not checking your tires at all.

Now how to know the accuracy of your gauge in day to day use. I suggest the following based on personal observation (no sponsorship or compensation received).

 Get two digital gauges. One can be like Accutire MS-4021B as seen on the right

or similar for your "master". The choice of the 2nd gauge depends on how you can access the valves on your rear tires.

I have hose extenders as seen below

that means I do not need the more expensive dual foot gauge like Accutire MS-5515B seen on right so I use two of the 4021B. If you don't have valve extenders your day to day gauge would be the dual foot gauge.
My master is packed away in a box so it does not get damaged or knocked around. Once a month I measure one front tire with both gauges and record the readings. The difference between the two readings should not change by more then 0.5psi from new.  If I notice a change in tire inflation not explained by significant temperature change or puncture I will compare the daily gauge against the master gauge. I figure the chance of both gauges going bad the same amount and in the same direction at the same time is vanishingly small.  The 4012 is available at $10 and occasionally for less than $7.

TPMS readings have not found to be as accurate as hand gauges. Their purpose is to be a warning of a loss in pressure. You should set tire pressure with the hand gauge. You should see a fairly constant difference between the TPMS and the hand gauge.

It is important to measure your tires when they are at ambient temperature. This means the air temperature. NOT exposed to the sun or even under a tire cover or having been driven on for at least two hours.

Tire pressure changes with temperature and you can use the rule of thumb of 2% for every 20F.

Save money at the fuel pump

Tire Pressure.
 Ya, we tire engineers are like a broken record when it comes to tire pressure.Tire Pressure Monitor System. If you don't have a TPMS then we tell you to check your pressure every morning you are traveling.

We tell you you need to monitor it all the time by using a
We also want you to get your RV weighed then to consult some confusing tables to learn how much pressure is needed for your motorhome.
To make things worse we expect you to have an accurate gauge but then we don't tell you how to check your gauge to be sure it is accurate, or even why its important to have an accurate gauge.

Is that what's bothering you bunkie??

Well this post is going to help answer your questions, solve the problem of how to be sure your gauge is sufficiently accurate and to top it off tell you that its possible that many of you will end up saving more money than the "fix" costs.

To start off lets take a look at accuracy of gauges many RV owners are using today.
In a previous post I showed the results of a series of gauge tests conducted at an RV Rally Sept 2012 on 11 gauges, we saw that there was a 45% failure rate. This was higher than the failure rate seen on a similar sample from 2011 when 11% failed. This year my test sample was 24 gauges and we had a 12.5% failure rate. Failure is defined as more than 5% off when compared to a certified digital gauge that reads to the nearest 0.5 psi. For all these tests the test pressure was between 77 and 95 psi.

According to sources there is a range of fuel economy savings possible from keeping your tires properly inflated because tires are only one of the contributors to fuel consumption. Also different tires have different affects on fuel economy based on a number of variables. Some of these include different rubber compounds, basic tire construction features, tread design and tread depth. Without getting technical we learn that the impact of lower inflation on mpg ranges from  0.05% to 0.3% per psi. Now you may say this isn't much but lets look at how this hits you in your pocketbook

Assume fuel is $4.00 a gal when you do a fill-up. Lets look at the range of savings at -2psi and -10psi from our goal of 100psi (our cold inflation to carry the load with a built in +5psi safety factor). With a 30 gal fill-up the range of "extra cost" for being low on inflation is between $0.12 to $3.60. If you do a 70 gal fill-up running low on air is costing you an extra $0.28 to $8.40 for that fill-up. Remember this is from just being 10 psi low on your tires. A number of the gauges we tested had the owners running from 9 to 18 psi low so they were wasting some real cash.

With this much potential loss from your pocket each fill-up you certainly do not want your pressure gauge to be giving you wrong readings. Having an accurate gauge is easy and based on the above calculations you might save enough in a few tanks of fuel to pay for what you need.

A quick search for "Digital tire gauges" at Amazon. shows you could spend well over $300 for a digital gauge but that really isn't necessary. You can get a good digital gauge for less than $10. This becomes your "Master Gauge" Then you get a second gauge for every day use. When the gauges are new you compare them and they should give the same reading. If you notice a sudden change in tire inflation pressure you can dig out your master gauge and confirm. You should also do a comparison at least a few times a year. The chances of both gauges going bad the same amount at the same time is vanishingly small.

Finally don't just throw your master gauge in the bottom of your tool box. I suggest you keep it in a safe place that offers some type for protection.

Should I jack up my RV?

I see this question come up each year when some people are parking their RV for the Winter. There is a quick and simple answer and a more involved answer,

Quick Answer:
I do not jack up my Class-C RV over the winter but I do inflate the tires to 80 psi (the pressure on the tire sidewall). I do cover the tires with white covers over the winter (and whenever I camp at a location for more than 1 day).

There are other things you can do such as keeping the tires out of contact with wet sand and dirt. I covered this with pictures in my post on “Winterizing” 

More involved answer:
Jacking up the RV and removing the tire is one of those things that in a perfect world would be easy to do but in reality it isn't as many RVs don't have good jacking points or are so heavy you need very HD jacks so it may just not be safe to do..
I do understand the theoretical damage done from long term parking in one spot but again theory and reality clash. Lots of actions can theoretically prevent microscopic problems but if you extend the life of a tire by 1 month if you were to jack it up every time you were planning to park for more than 3 months, would it be worth the effort? Probably not.

There are actions that are easy to do and relatively inexpensive that can provide real "bang for the buck". I have covered these in various posts on my blog but will touch the big ones here.

1. Your actual static load should not exceed 85% of the max load capacity on any individual tire for the inflation you run..
2. You need to weigh and learn the actual load, when fully loaded and not simply take the total axle load and divide by the number of tires. You will NOT get the correct tire load by doing the simple division. You can download a worksheet on how to do the math HERE Or HERE

3. Get a TPMS so you will get a warning when (not if) you start to loose air due to puncture or leaking valve
4. Cover your tires with WHITE tire covers if they will be in the sun for better part of the day. A couple of hours in full sun does about same damage as a full day in the shade in Phoenix.
5. Replace the snap-in valves or rubber parts of your bolt in tire valves whenever you buy a new tire.
6. Get and use a digital hand held gauge at least once a month, even if you have a TPMS. You will probably be adding 1 to 3 psi each month anyway to maintain the tire inflation. This will also serve as a check on the TPMS.
7 NEVER drive on a tire that has lost 20% or more of its air. Structural damage will be done. Such damage is cumulative and this damage does not repair itself. (See post on Potato Salad)

8 Do not believe everything you read on RV Forums. Ask the poster for their actual training in failed tire analysis. Simply having owned tires from company XYZ does not make that person a knowledgeable source.
9. If you have a multi axle trailer your cold inflation should be the inflation on the tire sidewall

10 You might put on your To-Do list to read the posts in my blog (including the ones where I later point out my errors), then you could subscribe so you get a notice when I do a new post. I do about 3 a month so you will not be overloaded.
11.If you have a multi-axle trailer, learn how to do a rotating tire inspection and do your tires at least once a year. Watch this VIDEO  read the background in this POST to see what a failed tire looks like before it comes apart.

Gauge Accuracy - I was wrong.

Previously in my post on gauge accuracy I suggested that you could get your pressure gauge checked by visiting a tire store from a tire manufacturer or very large dealer. Well after doing more investigations it turns out I was expecting more from dealers than they actually provide.

Tire dealers are primarily in the business of selling tires and vehicle service. They use gauges every day to inflate hundreds of tires but they do not have ISO calibrated gauges as this would be very expensive and the difference between the gauges they use every day and a calibrated gauge might be measurable but is not really meaningful.

Now this doesn't mean you should not make an effort to confirm your gauge is reasonable accurate the question is "How accurate is accurate enough"? If your gauge is +/- five percent of the inflation pressure you're measuring then you are probably okay, especially if you're running at least five percent above the minimum inflation needed to carry your load as measured in individual tire load scales.

Still, I would suggest you get a digital gauge for your personal "master gauge." They cost as less than $15. Then use that master gauge to check your everyday gauge. Keep your master packed away and not rattling around the bottom of your tool box under the hammer and wrenches where it will get dirty and damaged.

If you check a front tire with the digital master and then your every day gauge and note the difference then when you compare them again maybe a month later you should see the same difference. It is very unlikely for both gauges to go bad the same amount in the same direction at the same time.

If both gauges are within two to three percent when you first get the new digital master you are probably okay. If there is more than a three percent difference then I would check both against some other gauge to learn which is wrong.

Hope this clears things up and sorry if I mislead anyone.

Be Prepared

I am pretty sure the Boy Scouts were not thinking about RV tires when they selected the motto of "Be Prepared".
However that motto does provide good guidance for RV owners when it comes to their tires and being prepared in the event of a failure or even better yet being prepared for travel in a way that can decrease the chance of having a failure.

Right now most RV owners either have their rig parked under cover for the winter or are keeping relatively warm in our southern states. In either case there are not many RVs on the road this time of year. But in another couple of months it will be time to get the cover off the rig, dig out the campground guides and start planning their adventures.

I would like to offer some suggestions on a few things to do that can make your life a bit easier when it comes to maintaining your tires and steps to take that may possibly save you grief down the road.

1. Have your tires inspected
       You should be able to find a dealer for your tire brand that is willing to give your tires an external visual inspection. I covered this in detail in this post.  They should look for bulges in the sidewall or an out of round tread. Cracks or cuts that exceed the specifications provided to the dealer by the tire manufacturer of your brand of tires. I believe that all competent and properly trained dealers should have a guide with specifications on what would be considered an "adjustable" level for depth or width of crack or cut. Even if the tire is out of warranty those levels would be a good guide for what the manufacturer considers safe or not.

2. Inflate the tires to the level required for your unit.
     For trailers this usually means to the inflation on the tire sidewall. I covered my suggestion for trailers in depth in a post just for trailer owners. I would include all towable units, even dollies in this category. For motorized units this would probably mean the minimum needed to carry the highest expected load plus 5 to 10 psi "cushion". You can refresh your memory about the importance of proper inflation here. If you haven't had your RV weighed and calculated the individual tire loads I strongly suggest you read this post. You can check the RVSEF web site for the schedule of locations they will be doing RV weights. Here is a site with downloads and worksheets on how to calculate actual loads.

3. While at the tire dealer confirm your tire gauge matches the calibrated master gauge the dealer has.
     It doesn't do you much good to check your tires if your gauge is off by 10%. I covered gauge accuracy in a post and you can see the high failure rate observed based on actual calibration checks..

4. Record your full DOT serial, including the date code.
     If you write this information in your record book and file it along with other important papers such as insurance and registration you will only need to do this once in the life of the tire. If you have this you can easily calculate the tire age and even check if the tire is on a recall list in the event of a recall or safety campaign.

5. Install your TPMS and confirm the batteries are working.


I hope you all have a safe and enjoyable 2013 RV season.

Gage Accuracy, Rotation, Alignment, Fuel Economy

Went back and tried to review comments and questions. I know some have been answered in depth but some others maybe not enough to make the point clear so here are some short replies and thoughts.

May 15 Quampers asked about tire gauge accuracy.
Quampers, I currently travel with two digital gages that are accurate to 0.5 psi when compared to ISO certified pressure gauge in a tire testing lab. I can compare readings between these two gages and as long as they are within 1.0 psi or less of each other I can be pretty confident they are still giving me the correct pressure. I will have the gages checked on a two year cycle even if I find no change in their readings. These are the gages I will use to compare to others at my Tire Basics seminars. FMCA at Madison, WI Aug 11 and at Gypsy Journal Rally at Celina, Ohio Sept 27.

June 25 Tom asked about tire rotation and where to put the newer tires if you don't buy a complete set.
I have heard it said by others that if you don’t have a problem with irregular wear on your RV you don’t need to do an alignment or rotate tires. I guess that may be true as long as 1. The manufacturer of the coach bothered to do an alignment at the end of the assembly line and 2. You keep a close eye on the wear of your tires.
Now I can tell you that in my experience it appears that some manufacturers may be cutting corners when it comes to spending the time to set the alignment. After all your warranty is only good for a year and in that time most RV drivers will not get enough miles on their tires to see alignment problems. The second point is that even if you do complain the mfg will probably tell you it is your responsibility to ensure the coach or trailer is in align, even though there is no mention that as a new owner one of the first things you would need to do is to drive your new rig to a heavy duty alignment shop and do the work the manufacturer should have done in the first place.
When it comes to the question of where to place the new tires there are a couple of things you must consider. If your RV has dual tire position you need to be sure the Outside Circumference difference on a pair of tires in dual position is ¾” or less otherwise there will be more load transfer from one tire to the other than is advisable. Not Outside Diameter buy OC which is a more accurate measurement to get if you don’t have special tools. So for Tom it will depend on how many tires he needs to replace as to where they go.
When I recently rotated my tires because of miss alignment on fronts.
They looked like this in inner shoulder and this on outside shoulder













I measured all seven tires (including spare) then put the most worn tire as the new spare. It worked out that the still new original spare ended up on the front and I was able to match my duals to within ¼” for each pair.

Now if we are talking about a passenger car the two new tires should go on the rear to give you better wet traction and decrease the potential of a rotating skid and spin-out in an emergency. This is less of an issue for heavy RVs due to the weight cutting through the water better.
On multi axle trailers I would put the new tires on the front as this location is less likely to suffer a puncture and if you are going to lose a tire due to puncture I would rather lose the older tire. Hope this didn't confuse too many people. As you can see there are a number of things that need to be considered.


June 29 Bob asked about LRR or Low Rolling Resistance
LRR spec tires are designed to trade off a number of performance characteristics to improve fuel economy. Rolling Resistance is the force it takes to roll a tire under load. The more force the more energy needed to make the vehicle go down the road. I have not heard of LRR tires in RV sizes yet as most of this work is aimed at Hybrid cars getting 40 MPG or more.