Reply to "How fast am I going?"

SD Steve (Guest)

Physics? Just a little math�

Tire height is approx: 15" + (345 * 40% * 2)/25.4 = 25.87" (25.4 is mm per inch)
So circumference is approx: 3.14 * 25.87" = 81.23" (3.14 is Pi)

So speed in MPH = RPM * 81.23" / gear ratio / final ratio * 60 / 63,360

Where:
60 is minutes/hour
63,360 is inches/mile

So, if 5th gear is 0.67 (I am making this up) and final ratio is 4.22 (I think that is right) and we use 2,500 RPM we get�

2,500 RPM * 81.23" / 0.67 / 4.22 * 60 / 63,360 = 68 MPH

Seem about right? Hope that helps.

1/25/0211:50 AM

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