I recently got a PM from a member who will remain anonymous asking me how to calculate the vehicle speed given the engine RPM, transmission gear ratio, final drive ratio, and tire diameter. I figure I might as well post this for all to see. I realize that this doesn't belong in OT, but I'm too bloody tired to go somewhere else, and, after all, the moderators need to feel needed anyway.

Let
A = engine speed (RPM)
B = gear ratio
C = final drive ratio
D = tire diameter (inches)

Then,

A is the crankshaft speed (RPM)
A / B is the driveshaft speed (RPM)
A / B / C is the axle speed (RPM)
A / B / C * pi * D is the vehicle speed (inches per minute)
(pi*A*D)/(1,056*B*C) is the vehicle speed in MPH.

As an example, let
A = 6,000 RPM
B = 0.86:1 (overdrive)
C = 4.10:1
D = 24 inches

Then the crankshaft speed is 6,000 RPM,
the driveshaft speed is 6,000/0.86 = 6,977 RPM,
the axle (and wheel) speed is 6,977/4.10 = 1,702 RPM,
the vehicle speed is 1,702*3.14*24 = 128,301 inches per minute,
the vehicle speed is 128,301/1,056 = 121 MPH.

Speed = (3.14*6,000*24)/(1,056*.86*4.10) = 121 MPH

Time for bed.

 

Bill-

170 -- I've done it twice! UCISteve claims 4% speedometer error, but that's what I saw on my dash.

G