What inflation to run when changing ply rating or tire size?

RV Tire Safety
with RV tire expert Roger Marble

Following a few different RV forums, I see a number of people that clearly do not follow this blog and have questions on what inflation to run. While many of these folks are discovering that the new Goodyear Endurance line seems to come with higher Load Range or only in larger sizes, they end up not knowing what to do about inflation.

In one case the person didn’t even know they were buying a larger size, so I do wonder sometimes how people managed to ever buy tires for their car if all they seem to know is that tires are round and black and seem to cost too much.

When you replace tires on any vehicle, you should always be sure the load capacity of the new tires is equal to or greater than the capacity of the original tires on the vehicle.

Regular readers of this blog know that load capacity depends on BOTH tire size and INFLATION level. You also know that there is a “Tire Placard” or “vehicle certification label” on or in all your vehicles that identifies the original size tire and the inflation recommended by the manufacturer. In the case of RVs, the label also should tell you the GAWR, which is the maximum load you should ever have on each axle confirmed by scale readings.

So back to the original question of what inflation to run.

Example 1:  There was a change in tire size but the Load Range stayed the same.

Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-C.

With a quick check of a Load & Inflation table for ST type tires, we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can support 2,150# @ 50 psi. This is a nice 18% increase in load capacity. I would suggest that in this case the owner continue to run 50 psi and enjoy better tire life.

Example 2:  There was a change in tire size AND in Load Range, as can happen with the new Goodyear Endurance.

Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-E.

Back to the Load & Inflation table for ST type tires, we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can now support 2,830# @ 80 psi. This is a 55% increase in load capacity. If the owner were to run the 80 psi as indicated by the tire sidewall, the “ride” would probably be hard on the TT. In this case I would not follow the inflation on the tire sidewall. I would suggest that in this case the owner continue to run 50 to 55 psi and enjoy better tire life. In this example the owner expressed concern about running lower inflation than marked on the tire. Some people even incorrectly said that running less than 80 psi would somehow overheat the tire. I pointed out that I saw no problem with running less than the sidewall inflation as long as the actual load had been confirmed with actual scale readings and that there was a good margin of capacity over the actual loading.

Example 3: A Class A owner wanted to change from a Michelin 275/80r/22.5 to a Firestone FS 591 295/75r/22.5. His reasoning was twofold: First he was concerned about the historical reports of  delays in getting the sometimes hard to find Michelin size, and second was the significant one for him – cost difference.

Both tires were LR-H. The owner did a good job of consulting the tire manufacturer’s data sheets to learn the actual tire dimensions. One poster had offered a generic tire size comparison webpage but the numbers the page generated were clearly different and could lead to problems. 

The load capacity for the Michelin and Firestone tires is 7,160 @ 120. The owner had also confirmed the dimensions were acceptable for the application.

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

##RVT840