Engine speed measures how quickly the gears rotate, in revolutions per minute. Speed, however, measures how fast the car travels, in miles per hour. Gear ratio relates the two, but to calculate gear ratio the radius of the tire must be known.
Things You'll Need
 Calculator or computer.

Convert speed to inches per minute. To do this, multiply speed in miles per hour by 1,056. For example, if speed is 40 mph:
40 x 1,056
= 42,240 inches / minute

Divide the result by pi.
42,240 / 3.14159
=13,445.4209

Divide the result by 2.
13,445.4209 / 2
= 6,722.71

Divide the result by the tire radius. For instance, if the radius is 9 inches:
6,722.71 / 9
= 746.97
This number is the axle speed in revolutions per minute.

Divide engine speed by this answer. For instance, if the engine speed is 4,000 RPM:
4,000 / 746.97
= 5.35
This is the gear ratio.
Tips & Warnings
 If a calculator is not available, enter the formula into a computer's calculator or a Google search bar.
 This formula assumes a single gear ratio for a vehicle. If multiple axles have different gear ratios, they cannot be determined using speed, engine speed and tire radius.
References
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