Popular Automotive Math Formulas and Conversions

Automotive Math Formulas

RPM = MPH X Gear Ratio X 336 divided by Tire Diameter (inches)

The rolling diameter of a tire differs from the standard diameter. Measure rolling diameter by marking rear tire and ground with chalk and driving the car forward one tire revolution. Divide measurement by Pi (3.1416)

MPH = RPM X Tire Diameter (inches) divided by Overall Gear Ratio X 336

To convert from:

  • Miles to Kilometers, Multiply by 1.609344
  • Liters to Cubic Inches, Multiply by 61.023744
  • Inches to Millimeters, Multiply by 54.4
  • Millimeters to Inches, Multiply by .0393701

Displacement: (cubic inches) = Bore (inches) Squared X Stroke (inches) Xnumber of Cylinders X .7854

Displacement: (cubic inches) = Pi (3.1416) X Rdius of Bore Squared X Number of Cylinders X Stroke (inches)

Rear End Ratio = Flywheel Turns X 2 divided by Rear Wheel Turns

Jack only one rear wheel off the ground and shift transmission into high. The formula applies only to standard three-speed transmission that has high gear as a straight-through drive. If the car has overdrive, lock it out. Rotate wheel one full turn and count flywheel turns to the fraction.

Transmission Ratio = Flywheel Turns X 2 divided by Rear Wheel Turns X Rear End Ratio

Shift transmission into either first, second, or high gear with overdrive, depending on which gear ratio you want to find. Follow the procedure for the “Rear End Ratio” formula above.