Glenn

Does anyone know the formula for proper tire pressure for over sized wheels and tires. I know if you have a 10.5 inch wide rim with a 275/35/20 it gets more than 32psi. So what is the formula?

kaylawarner

Tire weight/tire pressure = Load capacity pounds per psi
Most tires drop about 1 psi of pressure monthly. Changing seasons will also reduce tire pressure, usually at the rate of 1 psi per 10 degrees of temperature change. For example, a tire that reads 31 psi at 30 degrees might read 35 psi at 70 degrees. The only way to address these changes is to check your tire pressure often and at regular intervals. You will be rewarded for your efforts with better fuel economy and more even tread wear.

Stevedotmil

I don't think that it really matters with the change in size. The outside temp is more important. If you check the tires to see what the max pressure is and then go 75% of that to account for the rise in pressure once the tires are hot then you should be fine. I check mine once a week because of the crazy weather where I live.

Glenn

I'm pretty sure the rim width and tire size are part of the equation?

Siber Express

I never actually did anything involving width of rim size, I start at 35 PSI which is about 80% of a normal tires max of 44 PSI and then I adjust for handling characteristics. On my 255/40/19's I am running 38 in the fronts and 36 in the rear. Like Steve I check my tires frequently not going more than 2 weeks.

Stevedotmil

Yeah, I'm almost never running the recommended tire pressures due to weather and what the car feels like when I'm driving.

southcarolina12

ive had two trucks with 35x12.5 tires on them and ive ran the pressure that it says on the inside of the driver door and they all wore perfect. id imagine it would be the same with a car.

El Coyote GT/CS

I don't know the formula for "proper Inflation" but I know what I would do to calculate a starting point.

The assumption is the rim size is proper for the tire size, so I don't care about either of those. Observe the max load and max pressure printed on the tire - that should be considered 100%. Figure out what the actual load on the tire is when the car is fully loaded. Remember there are 4. Consider any significant weight distribution differences. Divide the actual load by the max load to get the % difference, then multiply the max pressure by that % to get the required air pressure to carry the load. Adjust that pressure as needed for any temperature variations.

From there, you have to drive around and mess with the pressure to dial them in. Watch the wear/scuff patterns too. Avoid crashing!

And beware of special purpose tires! This method may not be appropriate for them.