yeah I would strongly go with Jerry, I went from a 235/75/15 to a 31x10.50x15 and got My RPMs down. At 75 I sit at little over 2300 RPM, and In some cases with a head wind and a 16 foot boat trailer with boat, it lugs it down and have to go back to 4th. it still gets reasonable fuel mileage like 21 to 24 MPG on regular unleaded with my current tire size. I want to see how it does with mid or even premium but gas here in Wyo is still 2.72 a gallon for reg and 3.10 for premium.

89'Isuzu Trooper LS 5 spd 2.6L
31" Big O X/T's
15x8 Eagle 058's
K&N Filter w/ Air Box Mod
2.25" DynoMax SuperTurbo
PolyUrathane/Indy4X Bushings
Suspension/ FrontEnd Tech/Hunter Certified Alignment Tech.