I have actually experience this once, changing my Jeep rims. The problem is manly the tire size. These tires are about 30% larger circumference than the stock tires. This is like starting out in second gear would be with the stock tires.The only way to compensate for that is to change the axle ratios by the same amount. If you have a 3.72 axle ratio (just a guess) you will need to change the gears in both axles to about 4.84. You will need to go to an off road specialty store they will be able to set you up with the correct gears and probably install them for you. The larger tires make the engine work harder because they roll harder so you will probably still get poor mileage but getting the gearing right will help a little.