# 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).

• 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.

## References

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

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