I just installed my 3.55 posi and broke in the gears. Then I thought I would go on my favorite little route on a limited acess road where there are no cops and give it a real test. the rear gear is great and I loved it. But I know my speedo was wrong. It said 120mph. I think I was doing 90. So how may teeth should I get on my speedo gear, if you gents know.
I think this is the formula: Take the tire diameter(in inches) multiply by Pi (3.1416) to get the circumference. Divide that into 63,360 (inches per mile) to get the tire revs per mile. Then Multiply the drive gear tooth count (C-4 is 7 teeth) X axle ratio X tire revs per mile, then divide by 1000 to get the driven gear tooth count. The tire diameter can be calculated with the sizing on the sidewall. First number is the section width in millimeters, second number is the aspect ratio. Multiply the section width X aspect ratio ( we'll do a 275/60/15) 275 X .60 =165 This is the sidewall height. Take this times 2(top and bottom of the tire) and divide by 25.4 to get the sidewall height in inches, add that to the wheel diameter to get the total diameter. (165 x 2 = 330, 330/25.4 = 12.99, 12.99 + 15 = 27.99, round that off to 28. Does this sound right?
