18 inch tire pressure

The GS-D3's? Man, I really, really wanted those.

Air pressure, under what the Max. pressure is, which on those, is 50psi, so try like 40psi and go from there.

Careful.. There's usually 2 Max pressures. One my Falkens 245/40/18 it says MAX 50 then right on the sidewall where it shows the specs it says do not fill past 40 PSI. I run 35.. I think anything over that is overkill.

Rule is, the vehicle determines the air pressure......
I have 18" Bridgstones, and the Max pressure shown on the sidewall is 50, the actual pressure is 35.

50 will make it feel like you are on steel wheels w/ no rubber, if they are a lower profile.

Max pressure is just a warning if you need to compensate for a 24' Bayliner hitched on your rear bumper.
And I don't think you are planning on doing that......

I was running 30 but it felt like the tire would come off the rim when cornering, I've got 40 in at the moment bit that feels a bit harsh. Sounds like 35 is where I sould be at.