Mine cruises comfortably at 80-85 the thing just purrs at that speed. No lift yet though. If it still handles like this after lifting it I will be a happy camper. As far as differential gearing if no tags are there or readable its simple. make a mark on the tire and the body with some sort of marker or crayon do the same on the pinion yoke and the diff housing then turn the wheel one turn and counting the number of turns of the driveshaft/pinion yoke is pretty accurate and simple.
Art