Define Gearbox Ratio

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In short, the ratios of a transmission's gears define its ability to convert engine RPM to acceleration. These ratios play a role in top speed, acceleration, fuel mileage and even engine longevity.

Definition

  • A gear ratio is defined as the difference between input shaft (engine) speed, and output shaft (drive-shaft) speed.

Purpose

  • Transmissions have low first gears that permit the engine to spin faster than the tires, which increases its mechanical advantage and increases acceleration.

Ratio

  • The gear's "ratio" is the number of teeth on the output-drive gear compared to those on the input shaft. If a transmission has an input shaft with 20 teeth that meshes with a 60-tooth gear on the output shaft, then its ratio is 3 to 1 (60 divided by 20).

Ratio Spread

  • If a transmission has many gears that are very close in ratio, it is called a "close-ratio" transmission. This gear arrangement increases acceleration in vehicles with high-revving engines.

Overdrive

  • An overdrive gear is one that uses a smaller gear on the output shaft than that on the input shaft. This causes the drive-shaft to spin faster than the engine, which increases fuel economy under cruise conditions.

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References

  • Photo Credit Image by Flickr.com, courtesy of Reg Mckenna
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