For anyone who might read this, I'll point out that if you don't have a sure-grip differential, you must either turn both rear wheels together, or hold one wheel fixed while turning the other. With one wheel fixed, the pinion gear only makes half as many turns for each turn of the single wheel. For example, if one tire rev gives you 1.5 pinion yoke revs, then your ratio is 3.
Also, for more accuracy turn the tire several revolutions, count the yoke revolutions, and divide by the number of times you turned the tire. Example: 10 tire revs, 27 pinion revs => your ratio is close to 27/10 or 2.7 (now I'm assuming both wheels turned together).