305/40/18 Nitto 555R drag radials on a 9.5" wide rim, what psi should I be running on the street? Only got 2yrs and about 5k miles on my last set, 285/40/18 running 30 psi on the street.
Separate from the whole idea of driving around on DR's . . . keep a close eye - with a tread depth gauge - on center tread wear. Even at 28 psi I'm betting that the center ribs will wear a good bit faster than the outermost ribs.
I agree with Norm. When looking at the tires after driving over sand, I got a clear picture of the contact patch. The 245/50/16s I had at the time mounted on 16x7 V6 wheels did not appear to make full contact any higher than 20psi. You are giving up a ton of load capacity by lowering air pressure but there is plenty of wiggle room with a 97# load index and higher.
27 or perhaps 26 would be a little better for evenness of wear (as long as it doesn't get too "woozy-feeling" in the corners or at high speed). A 305/40-18 SL tire still has more load capacity at 26 than the non-GT500 OE tire sizes do at 32 . . .