Is there any baseline rule for adjusting tire pressures when you go plus 1, plus 2, or more? Do you just start with what the factory suggests for your original size and then adjust by the seat of your pants, or is there a more scientific way of going about it?
I typically start at 38 cold, as the specs on the door are for factory tires on a factory car. Adjust as necessary.
Yes, it's more car, tire, and use dependent than any general "rule of thumb" or taking info off the door jamb could provide.
But yeah, if you're starting from scratch, somewhere between 35-40 cold is a great place to start as its high enough not to screw anything up but low enough to probably be in the ballpark.
There's no "rule" for changing pressures with wheel & tire sizes. Generally a lower profile needs higher pressures. So if you found a good pressure for your old tires, use that as a starting point and expect to go up.
FWIW, Porsche recommends the same pressures for Boxsters whether they come with 16, 17 or 18 inch wheels.
The older BMWs (pre EPA labels) had a tire pressure chart for all optional tires on the B pillar. Usually it was higher pressures for lower profile tires, but I never noticed a consistent formula.
On an Audi-
I start out at 35 for just about all of my vehicles and go from there. I ran the truck higher (40 psi) because it was usually hauling or towing something.
Working in a high end wheel/tire shop, I usually stick to the EPA placard unless we are talking small sidewalls and mud tires. Stance cars with stretched tires, I go no lower than 40, mud tires no lower than 35, absolutely nothing lower than 30 and keep the staggered pressures to the same gap. I will not go over a tire's max psi cold pressure(usually only an issue with M/T or A/T tires on HD trucks). I never go below the door jamb placard. I don't care if it's a '95 Explorer or a '15 Huracan, I stick to these rules.
Outside of that, I go with what feels good.
Grab two thin cutting sheets at the kitchen store. Get one clear and one pink or blue spray white grease quickly on blue mate it with clear and run it over with tire in question. adjust as needed to get pattern fully to show. or do a few hi-way blasts between exit ramps and use pyrometer. There is some fancy carbon like paper with color changing inks in a layer that used for this too but not cheap.
Anti-stance wrote: mud tires no lower than 35, absolutely nothing lower than 30 and keep the staggered pressures to the same gap.
Why's that, don't mud tires usually need lower pressures? I run 16 front 11 rear on my Samurai for street & rally, and I go even lower for mud pit & crawling events, although that's a very light vehicle. At 35 psi they'd feel like granite cylinders and I'd be driving on only the very center of the tread.
I know Wrangler guys usually run pressures in the mid-20s for street use with mud tires.
You'll need to log in to post.
