Gear Ratio Calculator
Find engine RPM at a given speed (or speed at a given RPM) from your tire diameter, axle ratio, and transmission gear, with the overall drive ratio. Calculates in your browser.
Engine RPM
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Show the math
RPM rises with the overall gearing and falls with tire size:
overall = axle ratio × gear ratio
RPM = mph × overall × 336.1 ÷ tire diameter
The 336.1 constant converts miles per hour and inches into revolutions per minute. Taller tires or a lower axle ratio drop cruising RPM. All math runs locally in your browser.
What this does
A gear ratio calculator finds engine RPM at a given speed (or speed at a given RPM) from your tire diameter, axle ratio, and transmission gear, with the overall drive ratio.
How it works
The overall ratio is the axle ratio times the transmission gear ratio. The 336.1 constant converts the units to give engine RPM from road speed and tire diameter in inches.
RPM = mph × overall ratio × 336.1 ÷ tire diameter
How to use it
- Enter the tire diameter in inches.
- Enter the axle and gear ratios.
- Enter a speed or RPM.
- Read the matching RPM or speed.
Understanding your result
Taller tires and a lower (numerically smaller) axle ratio drop cruising RPM and improve economy; shorter tires and a higher axle ratio raise RPM for more acceleration.
Example
At 65 mph with a 3.55 axle ratio, 1.00 top gear, and a 28-inch tire, the engine turns about 2,770 RPM.
Sources & methodology
Last updated .
Frequently asked questions
How is engine RPM calculated?
RPM = mph × overall ratio × 336.1 ÷ tire diameter (inches), where the overall ratio is the axle ratio times the transmission gear ratio. The 336.1 constant converts the units.
How do tire size and gearing change RPM?
Taller tires and a lower (numerically smaller) axle ratio drop cruising RPM, improving economy; shorter tires and a higher axle ratio raise RPM for more acceleration.
What is the overall ratio?
The axle (final drive) ratio multiplied by the gear ratio you are in. In top gear at 1.00:1, the overall ratio equals the axle ratio.
Do you store my numbers?
No. All math runs locally in your browser.
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