One approximation if it is a sure-grip rear is to jack car up get both wheels off the ground, put car in neutral, rotate wheel one turn, count number of turns of drive shaft.....approx 3 1/4 turns would be 3.23, approx 3 1/2 would be 3.55, OR look for a tag under a cover bolt, OR measure tire diameter, tachometer reading, MPH and do some math