I'd really consider 31x10.5R15 BFG KO AT's. These are considered the shortest tire for 3 and up trails. (Rating system 1-5) They will get you onto 80% of the trails out there and won't sap too much form the Jeep. With any change in tire size you'll have to recalibrate your speedometer (easy). You'll feel less torque with the larger tires since your effective gear ratio will change. Offroad, you'll notice little to no difference. Gas mileage shouldn't really change since you not lifting the Jeep and once the speedometer is recalibrate it will show the correct mileage. Uncorrected, you'll think you've lost 2-3MPG.