If you've only got one wheel jacked up, the spider gears double the speed of the ring & pinion. One way to get a more accurate guesstimate of the rear end ratio is to roll the vehicle forward with both wheels on the ground one turn of the rear wheel and count the number of turns the driveshaft makes. The best way is to drop the rear cover plate and look for the tooth count stamped on the ring gear- - - -10-39 for instance, would be a 3.90:1 ratio. 9-37 would be a 4.11:1, etc.
Jerry
