How can you tell the rear end gear ratio?

Simply by jacking both rear wheels off the ground,put it in neutral,handbrake off, and chock the front wheels, and then turn the rear wheel 10 revolutions, and count the number of revolutions the tailshaft makes. For example, if it makes almost 38 revolutions, the diff ratio is 3.78:1. Alternatively, If the diff is out and you can't find a ratio stamped on it,count the teeth on the crownwheel and pinion gears, and calculate the ratio from this.