Trying to work out what final drive ratio I have. Confused because my understanding is the Ital open diff had either 4.1 ,3.89 or 3.64:1. Can some maths guru out there please check my calcs. 13" wheels with 185/70 profile yokos gives 872 revs per mile ( checked on tyresizecalculator.com and by physical measurement of circumference ). 70mph = 4000rpm ( checked on calibrated speedo and gps). 4th gear = 1:1. So 70 miles = 240000 engine revs and 872x 70= 61040 wheel revs. This indicates a final drive ratio of 3.932:1 ignoring wheelspin and assuming tyre pressures ok. So, what ratio have I got and what am I missing