been reading thru some old posts and this is my understanding

the OEM specs is

32 PSI @ 90 load index and max tire pressure of 50 PSI

and let's say I switch to a 90 load index and max pressure of 45 PSI then I should lower my pressure to maybe 30 PSI

this is correct?

what happens if the load indexes are different, so let's say my next load index is 92 and max 45 PSI.

i'm trying to see if there's some formula for translating the OEM specs to a new set of tires with different load index, max pressure.

thanks

 

For track use, get a probe-type pyrometer and adjust the pressure to give a linear temp profile across the inside-middle-outside of the tire. If the middle is too high I=140, M=150, O=150, reduce the pressure. If the middle is too low I=150, M=150, O=160, raise the pressure. I=160, M=160, O=160 ; I=140, M=145, O=150 ; etc. indicate the proper pressure for your car on that course on that day. Camber determines the slope of the temp profile. Ideally, the tire should be the same temp across the width of the tread, but usually you will have some kind of gradient.

I don't know of any formulas to guess based on load rating and max pressure, and I doubt that those are the independent variables anyway.

I am sure that someone will give you some good "starting point" pressures. But I don't have any. What tires (sizes) are you running?

-Max

 

i'm more or less just curious how to translate the OEM recommendation to new tires - just for regular driving, not track, autox or anything special. i asked somebody at tirerack and they said just keep it at the OEM specs, but it just seems odd that 32 PSI on 225/50-16 would be also 32 PSI for 18 veyr low profile tires.

 

About Load Index: Load index is merely a value assigned to a tire that shows how much weight it is capable of supporting. Imagine the tires as balloons. The more weight you put in the car the more those balloons will get squeezed and the higher the internal pressure will rise; the tread will run hotter as well. Load index takes all these values into account and says "This tire can support x amount of weight safely". I think Tire Rack has a chart for what the load index values equate to in pounds. Differing load indexes have nothing to do with what pressure the tire requires, they merely represent how much weight it can safely carry without bursting or overheating.

The weight of the vehicle is supported by the amount of air contained in the tires. The more air the tire can hold the less pressure it will need in order to support the weight of the vehicle. If you had two tires of the same width and circumference but one was a 50 series and one was a 40 series the 40 series tire would require slightly more air pressure since there is less volume inside the tire.

Problem is when most people upgrade wheels they go to a lower profile tire AND go wider. Since they are adding width this in most cases gets very close to the amount of volume the original skinny, high profile tire held.

In the far majority of cases Tire Rack is right on; run the air pressure you ran before. Different tires like different pressures but that comes down to specific models of tire.

If you want to do better than that the only way is to go to a track with a stopwatch and pressure gauge or a pyrometer.

 

thanks damonb, that makes perfect sense.