Tire Pressure

I got a set of 235/75R15 Maxxis Buckshot Mudders today on my 15x8 Ravine Rims(5.5 BS), and I was wondering what P.S.I. I should run them at(guessing about 30, but the shop said 40 even though the site says max P.S.I. is 35)? I was also wondering what I could air down to on the trail. Thanks!

there's no magic PSI number.
You need to use chalk or something similar to check the tire's footprint.
http://www.nitrogentiremachine.com/proper_tire_inflation.htm

Pressures over the tire's limit are a bad idea. I like running 35 on the highway. The higher pressures run cooler, are more puncture resistant but ride harsher.

Yet, I have no idea what you can get away with on the trail. M/T tires are made to air down so I wonder if the tire maker would give you some guidelines?

the tire maker will not give anything other than the maximum, and most will void any kind of warranty if the tire has been run underinflated.
a 15x8 is kinda wide for that tire, so I would start high and then go down until you start having problems.
I'd start around 15-17 psi and go from there.
I've aired down my swampers only once so far, 9.5 tires on 15x8's, at 15 PSI they were aired down plenty, and the bead seemed to hold OK.

For road pressure, I've found that XJ's tend to rock sideways, as if the stabilizer bar is bad, when the pressure gets too high, so you might just keep an eye on that, and keep the pressure below that. I run at about 35 psi for 235/75 snows.

Thank you guys. Good thing it's supposed to be warm today.