When I run a non-spec tire on anything, I use the chalk method to figure out pressures. You use chalk to draw a heavy line across the tread of each tire, sidewall to sidewall. Then, drive the vehicle on a paved surface long enough to get them warm. Then, you check the chalk and see how much of it remains. If its only worn off on the center, you need less pressure, if it worn off just at the shoulder/sidewall, you need more pressure. You're looking to get even contact across the entire contact patch. I just did this on my Jeep YJ with 33s, I inherited the Jeep and the ride was terrible. I did some suspension work and it improved, but still wasn't as good as it could be. With this method, I dropped the pressure from the 38 that my late aunt had in it, to 26 with a huge improvement in on-road handling, and no effect on fuel economy.