Sooty Posted November 3, 2018 Share Posted November 3, 2018 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 Link to comment Share on other sites More sharing options...
Ged Posted November 3, 2018 Share Posted November 3, 2018 Did you measure the actual circumference of the tyre or the rolling circumference?Yokohama gave a rolling circumference of 1790mm for AO21R's Link to comment Share on other sites More sharing options...
Sooty Posted November 3, 2018 Author Share Posted November 3, 2018 Actual circumference of inflated tyre is 1850mm Link to comment Share on other sites More sharing options...
Ged Posted November 3, 2018 Share Posted November 3, 2018 You need to use the rolling circumference.The ratio's you've quoted should give the following speeds at 4000rpm4.1 = 65.1mph3.89 = 68.623.64 = 73.34This is for AO21R's, it may vary for other tyres depending on wall stiffness, and very slightly depending on tyre pressure. Link to comment Share on other sites More sharing options...
Sooty Posted November 3, 2018 Author Share Posted November 3, 2018 Thanks Ged. That suggests 3.89:1 which makes sense Link to comment Share on other sites More sharing options...
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