What is the difference between gear ratio and overall ratio?

Gear ratio refers to a single pair of gears (driven ÷ driving teeth). Overall ratio is the product of the transmission gear ratio and the final-drive (differential) ratio, representing the total reduction from engine to wheels.

Why does a higher overall ratio mean lower top speed?

A higher numerical ratio (e.g. 11:1 vs. 3:1) means the engine turns many more times per wheel rotation. This multiplies torque but limits the maximum wheel speed at a given engine RPM, reducing top speed while improving acceleration.

How do I find the tooth count on my gears?

Transmission gear tooth counts are listed in factory service manuals or on manufacturer data sheets. Counting teeth directly requires disassembly. Many online forums and parts catalogues publish tooth counts for common gearboxes.

Does this calculator work for automatic transmissions?

Yes, for the mechanical gear stages. However, automatics with torque converters introduce slip, and CVTs have continuously variable ratios rather than fixed tooth counts. The formula applies only to the locked gear-pair portion of the drivetrain.

Can I use this for bicycle gears?

Yes. Bicycle drivetrains use the same tooth-count division to determine gear ratio. Omit the final-drive input (set it to 1.0) if calculating only the chainring-to-sprocket ratio, or include a hub-gear ratio if present.

Performance & Engineering

Gear Ratio Calculator

Calculate overall gear ratio from transmission gear teeth and final-drive ratio.

Last updated: 25 June 2026

What this tool does

This calculator computes the overall gear ratio by multiplying the transmission gear ratio (driven gear teeth ÷ driving gear teeth) by the final-drive ratio. Enter the number of teeth on both gears and the differential ratio; the tool returns the combined reduction ratio in the format X.XX:1. The output represents the total mechanical advantage from engine crankshaft to drive wheels.

Inputs

Result

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Formula

G_{overall} = \frac{T _{driven}}{T _{driving}} \times F_{drive}

G_{overall}

Overall gear ratio

T_{driven}

Number of driven gear teeth

T_{driving}

Number of driving gear teeth

F_{drive}

Final-drive ratio

How Gear Ratio Calculator works

The tool multiplies two stages of reduction: the transmission gear pair and the final drive (differential). For any given gear, the ratio is determined by dividing the driven gear's tooth count by the driving gear's tooth count. That intermediate ratio is then multiplied by the final-drive ratio to produce the overall ratio from engine to wheels. A higher overall ratio means more torque multiplication but lower top speed in that gear; a lower ratio favours higher speed with less mechanical advantage.

The formula

Gear ratio = Driven teeth ÷ Driving teeth
Overall ratio = Gear ratio × Final-drive ratio

For example, 40 driven teeth ÷ 14 driving teeth = 2.857:1 gear ratio. Multiplied by a 3.9:1 final drive, the overall ratio is 11.14:1. This means the engine crankshaft turns 11.14 times for every single rotation of the drive wheels.

Where this method is most accurate

The calculation is exact for spur gears, helical gears, and bevel gears in standard automotive transmissions and differentials. It assumes rigid gears with no slippage, which holds true for mechanical gearboxes and solid axles. The formula does not account for torque-converter slip in automatic transmissions, continuously variable transmission (CVT) ratios, or frictional losses in the drivetrain.

What this tool does not do

The calculator does not predict vehicle speed, acceleration, fuel economy, or towing capacity. It does not account for tyre diameter, which is required to convert engine RPM into road speed. The tool does not verify whether a given gear ratio is suitable for any specific vehicle, engine, or driving condition. It performs only the mathematical multiplication of the two reduction stages.

Disclaimer

This tool is provided for educational and informational purposes only. It does not constitute mechanical advice, engineering certification, or a recommendation for any particular gear configuration. Real-world drivetrain performance depends on engine torque curves, tyre size, weight, aerodynamics, and road conditions. Always consult manufacturer specifications and a qualified technician before modifying drivetrain components.

Questions

What is the difference between gear ratio and overall ratio?: Gear ratio refers to a single pair of gears (driven ÷ driving teeth). Overall ratio is the product of the transmission gear ratio and the final-drive (differential) ratio, representing the total reduction from engine to wheels.
Why does a higher overall ratio mean lower top speed?: A higher numerical ratio (e.g. 11:1 vs. 3:1) means the engine turns many more times per wheel rotation. This multiplies torque but limits the maximum wheel speed at a given engine RPM, reducing top speed while improving acceleration.
How do I find the tooth count on my gears?: Transmission gear tooth counts are listed in factory service manuals or on manufacturer data sheets. Counting teeth directly requires disassembly. Many online forums and parts catalogues publish tooth counts for common gearboxes.
Does this calculator work for automatic transmissions?: Yes, for the mechanical gear stages. However, automatics with torque converters introduce slip, and CVTs have continuously variable ratios rather than fixed tooth counts. The formula applies only to the locked gear-pair portion of the drivetrain.
Can I use this for bicycle gears?: Yes. Bicycle drivetrains use the same tooth-count division to determine gear ratio. Omit the final-drive input (set it to 1.0) if calculating only the chainring-to-sprocket ratio, or include a hub-gear ratio if present.

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Sources & Methodology

Overall gear ratio is calculated as (Driven teeth ÷ Driving teeth) × Final-drive ratio. This method reflects the standard mechanical-advantage formula for multi-stage gear trains, widely documented in automotive engineering texts including the SAE Handbook and Bosch Automotive Handbook.

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