Using a gear/tire calculator I came up with an increase of 3.57mph going from a 235/75 to a 235/85, I guess if you need tires the switch is ok. But I sure would not buy tires just to get that. Maybe for the cool factor and extra height look into 19.5's?? If not there is also always a gear swap, I had 3.55's in my 99 5 speed and fuel economy was good and it pulled anything I hooked to it.