Speed versus RPM Calculator
 

Tire Expansion:

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.

Calculator Equations:

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 step by step:

If the engine speed (for this example) = 6000 revolutions/minute,

then the driveshaft speed = engine speed divided by tranny gear ratio R1 = ( 6000 / R1) revolutions/minute,

and the rear axle speed = driveshaft speed divided by rear-end ratio R2 = ( 6000 / (R1*R2) ) revolutions per minute.

(Note: the symbol * indicates multiplication, and / indicates division)

So, the rear tire will be making 6000 / (R1*R2) revolutions each minute, causing the car to move forward (2*pi*r) * ( 6000 / (R1*R2) )  inches/minute (where r is the loaded tire radius in inches, and 2*pi = 6.28).

That is, the car will be moving ( 6000 * 6.28 * r) / (R1*R2) inches per minute.

Since 1 mile = 5280 feet = 63,360 inches, and 1 hour = 60 minutes, then the conversion from inches per minute to miles per hour is ( 60 / 63,360 ).

So, if the engine is turning 6000 rev/min then the car must be going (60 / 63,360) * ( ( 6000 * 6.28 * r) / (R1*R2) miles/hour.

Rewriting that all into a tidy form:

(0.00595) * (RPM * r) / (R1 * R2) = vehicle speed in miles/hour

    where:
           RPM = engine speed, in revolutions/minute
           r = loaded tire radius (wheel center to pavement), in inches
           R1 = transmission gear ratio
           R2 = rear axle ratio
 

Example:

1) SCCA Ford Spec Racer -

   RPM = 6000
   transmission gear ratio R1 = 0.73 in high gear
   rear end ratio R2 = 3.62
   loaded tire radius r = 10.9 inches
  
The car's speed at 6000 RPM in high gear will be:

(0.00595) *  ( 6000 * 10.9) / (0.73 * 3.62)  = 147 miles/hour

 

Enjoy!

 

Last Updated: 29-Dec-2007