back when i had my 340 Duster i swiched from 3:23 to 3:55-and noticed no diference in mileage-UNLESS i drove the car harder. Then the mileage dropped about 1.5 miles per gallon. My 340 engine was prety much stock-except for headers-carb-and intake manifold. If i had it to do again-i would not change from the 3:23 gears-car ran aout the same with both sets of gears. But when i went to 3:91 gears-big drop in mileage.