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Thursday, September 14, 2017

Can I change from 275 to 295 size tires?

Read this post on an RV forum:

"I need 6 new tires for '04 Dutch Star 38' on a Spartan chassis. Michelin tire models and sizing has me totally confused. Current Michelin size is 275/80R22.5. Dealers claim this Michelin size is interchangeable to 295/75R22.5 from brands that don't offer the original size. They tell me there will be no ill effects on the drive line or instrumentation."

I was concerned with this possibly incorrect information. In this case the two sizes have the same load capacities so that was not my concern.

Some folks replied to the original question by suggesting he use THIS or similar web sites to learn about tire sizes. I did have a little problem of using such a "tire size" site as the information is aimed at car application and does not include special information regarding dual tire application. Also the resulting numbers may or may not match published industry standards.

I pointed out my concern about the Minimum Dual Spacing which is a published dimension in industry standards and has been covered previously in this blog.

Some folks reported that they have changes tire sizes and had no problems and implying that any change might be OK.

I provided this graphic to help people understand what this dimension is.




As you can see the wheel offset and even wheel thickness can have an affect on this spacing. I also pointed out this example.  275's have a MDS of 12.24 and 295's have a MDS of 13.19.
Let's assume that with your wheels and with the 275's you have a physical 1.00" clearance down at the bulge in the sidewall near the road. So if you don't change wheels and put wider 295's on the new clearance will be   1.00 - ( 13.19 - 12.24) or  1.00-0.95  or a final clearance of 0.05"which is clearly too small.

I hope that those considering changing tire size consider not only the load capacity, which is very important but also the dimensions.



Tuesday, September 5, 2017

Tire Terminology

Sometimes people use their own terms when asking a question. Some RV forums have people answering questions using incorrect terms which can mislead others who may be reading  the question or answer being offered. This is just a "Failure to communicate" which doesn't help anyone.

Here are some common terms with their correct definition or example of proper use.

Load Range - A letter code  E, F, G, H etc that established the inflation and max load capacity for a given tire size. (this replaces the old, out of date "ply rating")

GAWR  Gross Axle rating is the MAX design load capacity on an axle. You should never exceed that number

GVWR Gross Vehicle Weight rating is the MAX design weight of a vehicle. You should never exceed that number. Note that the GVWR may be lower than the mathematical sum of the Front GAWR plus the Rear GAWR

Tire Load capacity This is the number of pounds a tire can support at a specified inflation. The Load/Inflation tables establish the various steps in load capacity associated with a level of inflation. Sometimes people incorrectly use the term "Tire Weight" which is incorrect. A tire itself may weigh 45# but it may be able to support 2,800# when properly inflated according to the Load & Infl tables.


CIP  or Cold Inflation Pressure is the measured Psi in a tire when the tire has not been warmed from running or from being in direct sunlight for the previous 2 hours. Technically we are talking about the Cold Inflation Pressure whenever we are discussing tire "inflation pressure".

Hot Inflation Pressure. Sometimes we may want to discuss the pressure in a tire when it has been warmed from running or from being in Sunlight. If this is the case, the term "Hot Inflation Pressure" should be used to be sure all understand the condition of the tire being discussed. You should not be bleeding down the inflation of a hot tire except under special, specific conditions

TRA or Tire & Rim Association is the US group that published the Load /Infl tables for tires made for use in the USA. There are similar organizations with similar tables in Europe (ETRTO) and Asia (JATMA). The numbers of pounds (Kg) and the associated PSI(Kpa) are many times the same or slightly different. The difference is due to using SI or Metric dimensions and rounding and conversions between units. It is legal to sell tires in the US that were designed to ETRTO or JATMA standards but the tires must still have the symbol "DOT" and the individual tire serial code molded into the tire sidewall and must still be certified by the tire manufacturer to be capable of meeting all the appropriate DOT regulations.

DOT  U.S. Dept of Transportation. The regulatory agency that specified the performance requirements for tires intended for use on public roads.

"DOT Certified". This incorrect phrase is many times used by those that do not understand the regulatory process. The DOT does not certify any tire. It is the tire manufacturer that is responsible for "Certifying" that all tires they sell are capable of meeting the DOT Federal Regulations.

FMVSS Federal Motor Vehicle Safety Standards You can read Sub part B of these standards HERE.

Tandem axle. This is found on many trailers where each axle has one tire on each end of each axle. Here is a write-up on Tandem vs single axle trailers if you are not sure of the description.

Tire loads are ALWAYS based on the load on tires on a specific position ie RF, LR, RR or LR etc. Front tires are "Single". Rear tires on most Motorhomes mounted side by side are "Duals". The tables show load capacity for individual tires in "single" position or in "Dual" position. You need to pay attention as some companies publish their own tables and show Axle loads or axle end loads which can lead to confusion. Tires on one end of an axle do not "know" what the load is on the other end of the same axle is. RV companies however assume axle loads are perfectly balanced end to end at 50%/50%. This allows the RV company to select the smallest (lowest cost?) tires capable of supporting 50% of the published GAWR but if one end of an axle loads that tire to 55% or 60% of the GAWR we would be looking at a tire overload situation.

In trailer application with two or more axles they may be some load transfer between the axles such that the ends on the right side may share some of the side to side unbalance but I know of no case where all the side to side unbalance is shared equally between all the tires on one side of a trailer.

##RVT810