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Popular Automotive Math Formulas and Conversions
RPM = MPH X Gear Ratio X 336 divided by Tire Diameter (inches)
Rolling diameter of a tire differs from standard diameter. Measure rolling diameter by marking rear tire and ground with chalk and driving 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:
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. Formula applies only to standard three-speed transmission that has high gear as a straight through drive. If 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 procedure for "Rear End Ratio" formula above.
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