Turn the input shaft in each gear and count the input revolutions to what it takes to turn the out put shaft one revolution. This will give you a ratio for each gear, if it takes 3.75 turns on the input shaft for one revolution in the out put you have 3.75 to one ratio. etc... if you are only interested in the final gear ratio you can leave the drive shaft in and count the inputs to one revolution at the axle. Or multiply the 3.90 by the trans gear ratio if you prefer.