Mark the input shaft, mark the output shafts. Turn the input 3.9 times, and the outputs shafts should only rotate once, should be able to see the differance between 3.6 turns of the input and 3.9. The messy way is to take the rear cover off and count the teeth on the crownwheel and count the teeth on the pinion, divide the crownwheel number by the pinion number and that should give you your ratio