fORMULA

Sid

Sid

Well-known member
Anybody remember the formula to figure out speed with a given gear ratio,tire size,and r.p.m. Lets say no overdrive in high gear.. 2400 rpm 3.25 rear and a 28" tire.
 
Let's figure it out. 2400 drive shaft rpm/3.25 yields axle rpm. Axle rpm times tire diameter times 3.14 yields speed in inches per minute. inches per minute times 60 divided by 12 divided by 5280 yields miles per hour. So rpm x tire size in inches /diff ratio times .00287 yields speed in mph.
 
Years ago, Speedway sold a "magic wheel" chart to figure this. My old eyes have trouble lining up the little lines now, but if all you need is close, the number would be just under 65.
 
Sid the formula is;

RPM X Tire Diameter / Gear Ratio X 336 = MPH

Plugging in your numbers;

2400 X 28 / 3.25 X 336 = 59.34 MPH

Wanted to add; This formula is assuming you are in top gear with 1:1 ratio. If overdriven include O/D ratio.

2400 X 28 / 3.25 X 336 X .89 = 69.14 MPH
 
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