Gas mileage and top speed depend on gearing more than anything else.Most of the ADs were running 4:11 or lower gears,which means poor fuel mileage and a low top speed. To get decent mileage go to a higher geared rearend or an OD trans. If your gonna use your original 3 or 4 speed trans. with a 1 to 1 ratio to the rear,change the rear out to about a 3:08 set of gears. Or put in an OD trans,700R4 automatic or a T5 5 speed trans where your top gear is around .75 to 1 ratio coming out of the trans.
