I'm doing a 38/34 psi front/rear right now. The Tire Rack says to inflate to mfg recommendations, and a local tire shop told me ignore that and put some more air in them, he suggested at least 40.

The sidewall does imply that the low-profile tires are capable of higher pressure. But I remember when Mario Andretti was testing a F355 at first he asked the crew to bump the pressure up to 45 in the back from the mfg. recommended 37. Then after 2 laps he asked them to lower it to 34--which would allow for a better grip and increased contact patch.

I've seen people here say with the stockers to have the same psi all-around. Anyone experiment and find the best pressure for 235/40-18?

 

If the tires are not made to support 40psi I wouldn't do it. The tires need to be able to give with the road abuse they'll get around town. You're going to blow your tire on one good bump and destroy your rim when then tire gives way. 35psi is as high as I would go.

 

When I got my tires on my rims from the manufacturer (235/40 on 18x8) they came with 38 psi in them. The max pressure is 44 psi on my pirelli's, and they said that a good rule of thumb for low profile is to keep it 6-7 psi under the max. BUT, i heard conflicting suggestions since then.

So, I ran consistently 34 psi all around for the life of my first set of tires (50k miles on Z rated!) and didn't experience any unusual wear. Also, with low profile tires, the lower you go, I would imagine that it reduces the probability of rim damage from potholes.