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Monday, June 1, 2015

Can't find your Tire Placard? Here is some help.

As I review various RV forums I occasionally come across special problems. Here is an example:
Originally Posted by J View Post
"I'm new to motorhomes so bear with me. I don't have a sticker on the door jamb to tell what psi is recommended. The tires are 215-85-16, it's an 1988 ford E 350 20 ft. class c. The tires state max psi of 80 but I will have to replace them since I found out they are 10 years old. I wonder if someone could give a recommendation for pressures front & back? Also, a tire shop said I could use LT 245-75-16 instead since they are approximately the same diameter but slightly wider. Just have to make sure they are D or E load rated. I have seen that size on a 16 ft. moving truck before. What say you all?"
While J has the additional problem of not being the original owner so he doesn't have the manuals or original tire info from 1988 he still wants to do the right thing and to use the proper tire inflation for his application. While your size or RV may be different I think that you could follow the info here and resolve similar situation.
Here is what I suggest.
 +++++++
My Class-C is 21' that was also built on a "cut-away van" chassis so we probably have similar weights BUT just because I run xx psi is not the best way to learn what you should run.

I strongly suggest you download and use the chart starting on pg 55 of the FREE RMA guide. (Note they will send the link to the guide in an email)
I doubt you have a slide on your Class-C so you are probably reasonably balanced and probably within 48/52% side to side.

With your RV loaded as you would normally travel (Food, fuel, water, people, tools etc) get to a truck scale. Use these links to find nearest to you.

Truck Scales and Weigh Stations Locator for Truck Drivers and Trucking Companies at Truckstops

Locate truck scales, truckstops, truck service centers, and diesel prices

Get the individual axle loads then assume one side has 52% of the load. (NOTE if your RV has slides or is a Class-A you really need to confirm individual corner weights or assume a 45/55% side to side split if you only get individual axle weight readings) Using that load you can use the Load Inflation tables for your brand tire (list on THIS link)
You may be light enough to use LR-D but may find a better selection using LR-E. Either way, find the Minimum inflation that is rated to carry at least the load you calculated for your LT215/85R16 tires.

The LT245/75R16 tires suggested by the salesman needs 11.34" Dual Spacing which is a lot more than your OE LT215/75R16 that only need 9.88". This is the clearance between the center-line of duals and is controlled by the wheels. Tire to tire contact or "Kissing" can lead to tire failure so unless the tire store is willing to put in writing that the LT245's will not have dual spacing issues or they will replace the tires for free, I would stay with your current size.

I suggest your CIP - Cold Inflation Pressure) be the minimum inflation calculated above + 10% so you don't have to worry about the need to add air if there is a big temperature drop. Remember never exceed the "cold" inflation pressure rating for the wheel which should be stamped on the wheel or available from the wheel manufacturer.


For those that don't know "RMA" stands for the Rubber Manufacturers Association. This 100 year old organization represents tire manufacturers that produce tires in the U.S.A. RMA represents its members before federal, state and local government entities; develops safety standards for passenger, light truck and commercial truck tires; advocates for environmentally and economically sound scrap tire management polices aggregates data pertaining to U.S. tire shipments; and, educates consumers about proper tire care, among other activities. They have a number of informative publications available for free on their website.
I think the RMA RV tire guide gives a good overview on tire safety and proper practices.


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