One of the most useful methods of measuring a bicycle's gearing is "gear inches." Gear Inches refers to the equivelent size of a drive wheel if the same gearing was installed on a single-wheel pennyfarthing-type of bicycle. Not the most practical thing to imagine, but it works for comparing drivetrain systems.
It is easy to calculate: Multiply the diameter of the drive wheel (your bike wheel with tire) by the number of teeth on the front chainring, and then divide by the number of teeth on the rear cog.
Note: This number is NOT how far a wheel turns during one pedal revolution, often referred to as inches/feet/meters of development. This is calculated differently.
Let's calculate your bicycle's gearing
If you are running a singlespeed or fixed gear setup, use the "highest gear" input and ignore the "lowest gear" and "gear range" variables.
How does this compare to other bikes?
A typical road bike has 700c wheels with 23c tires, a 53/39 crankset and a 12-25 ten-speed cassette. Its tallest gear is 116 gear inches. Its lowest gear is 41 gear inches. It has a total range of 75 gear inches.
A typical mountain bike has 26-inch wheels with 2.0 tires, a 44/32/22 crankset and a 12-32 cassette. Its tallest gear is 95 gear inches. Its lowest gear is 18 gear inches. It has a total range of 77 gear inches.
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