Originally Posted by Q777
225/40 and 255/35 ZR 18 Falken tyres on an AU.
What tyre pressure should these be run at for normal street driving?
My first post!
When I purchased tires for my lowered '95 Ranger, I called nearly everyone in the tire business that I knew. There is no easy answer.
My own experience with this subject, been doing the car thing since '72, and never getting anything but standard responses from those that I thought should know, caused me to developed my own theories.
In the past and with older tire designs one usually obeyed the 'door sticker.' I've come to believe that this pressure is only for OEM tires the same size or close to that size bought from aftermarket suppliers.
It is the load ratings that are the limitation on safe pressures in regards to tire design!
A tire that has a max load rating achieved at say 40psi for example and is used at 30psi if a 'door sticker' is to be believed, has a decrease in that load rating of 1/4 of it's max rating. Now I'm sure this is on a curve of some kind as the load rating at 20psi might be only 1/5 of the max. but the numbers will suffice for this discussion.
The Sumitomo HZR IIIs on my Ranger have a max pressure of 51psi. I cannot remember the load rating and am too lazy to go out there and look, but that load rating is achieved at 51psi. Reduce the pressure and you reduce the load the tire can safely carry.
Given this a tire that has a higher max pressure should be run at a higher pressure than a tire with a lower max pressure and all this is tied to the maximum load capacity of the tire.
Throw in the actual load the car or truck puts on each tire, and this might vary considerably, and you've only begun to reason the answer to the question. The combinations are nearly endless!
Every axle must me independently evaluated as to the load it will carry and the PSI is the only variable the mechanic/driver can influence.
Now, after all that, I run mine at 40psi because I get a darn headache just thinking about this!
BTW, FIRST CLASS FORUMS!