There is a potential for error in these calculations with bias-belted tires due to centrifugal force expansion of the tire at high speeds, but that effect is generally negligible for radial-ply tires due to the circumferential belts used in their construction. To get accurate results, racers using bias-ply tires should check with their tire supplier to determine how much the tire radius will change at various speeds.
For those who are curious about the calculations, here are the gory details of the speed versus engine RPM calculations:
Each revolution of the engine is reduced by the transmission gear ratio, each revolution of the output shaft of the transmission is reduced by the rear-end ratio, and each revolution of the tire makes the car move a distance equal to the circumference of the tire. Pretty simple really.
Let's go thru the calculations to create an equation for the vehicle speed...
First, let's define the meaning of the gear ratios:
Transmission Gear Ratio (R1): denotes how many engine revolutions there are for each driveshaft revolution.
Differential Gear Ratio (R2): denotes how many driveshaft revolutions there are for each axle revolution.
Now, we'll derive the equation:
If the engine speed (for this example) RPM = 6000 revolutions/minute,
then the driveshaft speed is the engine speed divided by tranny gear ratio R1 = ( 6000 / R1) revolutions/minute,
and the rear axle speed is the driveshaft speed divided by rear-end ratio R2 = ( 6000 / (R1*R2) ) revolutions per minute.
(Note: the symbol * indicates multiplication,
and / indicates division)
Rewriting that all into a tidy form:
(0.00595) * (RPM * r) / (R1 * R2) = vehicle speed in miles/hour
SCCA Ford Spec Racer -
Last Updated: 3-Apr-2012