Gear Ratio Calculator

What is a Gear Ratio?

A gear ratio represents the relationship between the number of teeth on two gears that are meshed together. It tells you how many times the Driver Gear (Input) has to turn to make the Driven Gear (Output) turn once.

• Driver: The gear connected to the motor or engine (Input).
• Driven: The gear connected to the wheels or load (Output).

The Trade-Off: Speed vs. Torque

In physics, you cannot get something for nothing. Gears allow you to trade Speed for Torque (Power), or vice versa.

1. Underdrive (Ratio > 1:1)

This happens when a Small Gear drives a Big Gear (e.g., 10 teeth driving 40 teeth).

• Result: Ratio is 4:1.
• Torque: Multiplied by 4 (Lots of power).
• Speed: Divided by 4 (Slow).
• Use Case: 1st gear in a car, or climbing a hill on a bike.

2. Overdrive (Ratio < 1:1)

This happens when a Big Gear drives a Small Gear (e.g., 40 teeth driving 10 teeth).

• Result: Ratio is 0.25:1.
• Torque: Divided by 4 (Weak).
• Speed: Multiplied by 4 (Fast).
• Use Case: 5th or 6th gear on a highway.

Idler Gears: Do they matter?

Often, there is a third gear placed in between the Driver and the Driven gear. This is called an Idler Gear.

Does it change the ratio? NO.

An idler gear has zero effect on the gear ratio or the speed/torque output. Its only purpose is to:

1. Fill the physical gap between two gears.
2. Reverse the direction of rotation (so the output spins the same way as the input).

You can calculate the ratio by simply ignoring all idler gears and only comparing the first and last gear.

Common Gear Ratios