The manual says 32 psi for the stock tires 225/50s.
I've heard if you go to bigger tires, that the inflation pressure should be higher?

What would 235/45/17 and 255/40/17s need for good pressure?

And anybody know what the difference in air pressure would be on a tire mounted on a car, and with it unmounted?

 

manual? LOL. keep changing the presure and see what gives you the best grip. then you can fiddle with different presure in the rear vrs front for a more balenced car. the 32psi is a recommandation for good tire wear/ gas millage/ comfort.

 

A track trick is to mark the outside of your tires whre the sidewall and the shoulder meet with chalk in three or four places around each tire. Drive the car for a day exposing it to the type of driving and roads you desire the best performance for. when you pull in to "the pits" look at all the tires checking how much chalk has worn off the shoulders of the tread blocks. Too much off the shoulder and the sidewall means the tires are under inflated--the tire is "rolling over" too much too much left on the shoulder tread blocks and the tires are over inflated and not being allowed max footprint/traction. Try to find a pressure that is a good comprimise between ride and handling.

 

The 32 psi guideline is for normal street driving and refers to cold pressures. Proper tire pressures really depend on the type of driving you do.

If you do mostly interestate driving, you could probably decrease cold pressure to 31 or 30 psi all around to decrease the wear in the middle of the tires.

If you're autocrossing, you should increase the pressure to 35 psi and adjust as necessary to suit your driving style.

Out on the track it's best to start with 31 or 32 psi cold. Turn a few hotlaps, and check the tire temps and air pressure changes. If you're lucky to have tire temps, this info would be the best guide as to whether your pressures are right, or if you need to adjust them. If you have tire pressures only, adjust the hot pressure so it's within the maximum tire pressure rating stated on the tire's sidewall. If the hot pressure is within the tire's maximum pressure limit, then adjust the pressures to suit your driving style (understeer or oversteer).

As for upsizing wheels, and how that affects tire pressures. In my experience with track-only Hoosier R3S03s and SSR Comps, the 32 psi cold pressure was a pretty good starting point. Once the Hoosiers got up to temp, I had to decrease the pressures in the narrower front tires to increase front tire stick, thus decreasing understeer.