Tuesday, December 6, 2022


RV Tire Safety: Tire “reserve load” calculations


with RV tire expert Roger Marble

I ran across a post on tire Reserve Load or Reserve Capacity that suggested the RV owner had been given incorrect information. Here are the post and my reply.

Personally, I’d run LTs, simply because of their higher “reserve” capacity; upwards of 30% over the stated load. Given that STs have, at best, 10% (used to be basically 0%), you’re still in ST load territory, with a much better tire. Hell, we used to run our old 1/2t trucks with massive loads and just air up to 60-65 psi and go. Yes, it wasn’t very far, or very fast, but those tires still lasted 50-60k miles, usually with steel cord showing around the edges. 🙂 We’d then take them off and put them on a disk or trailer and use them until they sun-rotted.

My reply:
I think someone misinformed you about “Reserve Load.”

All tires have a stated load capacity, for example, “2,340# Max Load” molded on the tire sidewall at a stated inflation level such as”50″ psi.

“Reserve Load” is the difference between the actual applied load and the stated load capacity and is many times stated as a percentage.

Example: A vehicle is on weight scales and we learn that a tire has 2200# load on the tire. The tire has a load capacity of 2,750#. 2,750 minus 2,200 = 550#, which is 20% of 2,750#. It doesn’t make any difference what type tire we are talking about as the math is still the same.

Now, it is true that for a given set of dimensions, e.g., 235/75R15, the stated load capacity is different depending on the type tire and inflation level. P-type and LT-type and ST-type each have different stated load capacities at their stated inflation pressure. For this discussion, let’s keep inflation differences out of the picture.

Let’s look at a P235/75R15 at 35 psi that is rated to support 2,028#. (In a trailer application P-Type must be de-rated by Load/1.1, giving 1,842# capacity.) An LT235/75R15 is rated for 1,530# @ 35 psi and an ST235/75R15 is rated to support 1,870#.

BUT the “Reserve Load” calculation is still (Tire Load Capacity/Measured scale Load).

The 10% margin for trailers is the difference between the GAWR and the total capacity of the tires on that axle at their max load. I have posted in my blog some actual margins showing that many cars have load margins of 25% to 35%, while some RVs made before Nov. 2017, when RVIA changed the “margin” to 10%, had margins of tire capacity vs. GAWR as low as 1%.

Hope this helps.


Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.



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