working at a tire store, i can tell you that we always keep psi at what the factory recommends for all vehicles unless the customer specifies different. even with plus sizing, we go by the factory specified psi. if you're worried about the air expanding while driving, i would recommend you get nitrogen put in your tires instead. i run oversized tires in my Expo and also run about 40 psi all nitrogen and don't have any issues with fluctuation. i just made a trip from Memphis to Fort Walton Beach non-stop and had no driveability issues or tire pressure issues at all. i've always heard that running a little more air in your tires gives you better gas milage so i guess that's why i do it. good luck on your trip.