With 18's you probably have to do a trial and error test, I'd definatly start high, say 35psi, take a drive - it will be rough, you can't escape that now - drop it down in about 2psi incrimints going no lower than 25psi. large rims with low profile tires need to have a higher air pressure to lower the risk of rim damage from potholes. I run 36psi in my 17's on my civic, we have some pretty nasty roads with poor maintenence and I've managed to get through 6 years with oversized wheels, while I check out local clubs and people complaining about broken and bent wheels - probably running stock 26/28 psi in their tires. Not sure if anything I've said is factual(like most things I say), but it's just my experience.
On my trackick, when I had my 30's installed(on my 15" rims), the tire guy filled them to 32psi. what a nasty ride down the street(50km/h)ÂÂ
I pulled over and dropped them to 20psi, that was better, but still a little rough, so I dropped them to 16psi, much better, but now it's pretty active on the highway(100km/h), I'm thinking on going back to 20psi as it handled better at higher speeds(100km/h)
