Saturday, December 3, 2022


RV Tire Safety: What is a “C” tire?


By Roger Marble
There are a number of Class B motorhomes that are being built on van chassis. Some of these vehicles come with European “Commercial” tires (185/R14C). I had a discussion with one owner who wanted to find the Load & Inflation tables for a “C” tire.

Well, for U.S. tires there are books published annually by The Tire and Rim Association that have the tables for almost all tires in the U.S. In Europe they have a similar organization, the European Tyre and Rim Technical Organisation (ETRTO).

Both manuals have dimensions and load limits, but the ETRTO manual does not have tables as they expect people to inflate to the tire sidewall pressure or the pressure specified by the car or truck company. BUT they do provide a formula.

It is a bit involved, but I worked up a spreadsheet for a “C” tire that you can use if interested. You do need to know the Max Load for a tire and the inflation level required to support that load, but you can then work out the results for your application. (Click the image to enlarge.)

Tire pressure metric

Have a tire question? Sign up for Roger Marble’s new Facebook Group: RV tire news, information and discussion, hosted by and moderated by Roger. He’ll be happy to help you.

Read more from Roger Marble on his blog at or on


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Roger Marble
1 year ago

Just in case some want to duplicate the formula for a different size, please note the “-.8” in the formula is an exponent. Also, do the calculations in Kg and Kpa then convert to Si.

Pete Morris
1 year ago

The above chart, certainly cleared things up !!!

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