Gear Ratio Calculator

What Gear Ratio is for

Gear Ratio Calculator is the page to open when you need gear ratio from measurements you already have. It belongs with the other automotive calculators because that is the work this arithmetic supports. Type the entries first. The result stays hidden until you press Calculate, so a half-finished form never flashes a number you might mistake for the answer.

How to fill in this calculator

1. Input Gear Teeth. Enter the value this part of the job needs.
2. Output Gear Teeth. Enter the value this part of the job needs.
3. Input Speed (optional). Enter the value this part of the job needs.
4. Input Torque (optional). Enter the value this part of the job needs. The unit list on this field includes Nm, lb-ft, lb-in.

The formula in plain language

A gear ratio is the tooth count on the driven gear divided by the tooth count on the drive gear. Engine rpm at a road speed is mph times the overall ratio times 336, divided by the tire diameter in inches. A larger tire lowers rpm and makes a speedometer read slow.

A worked example

A ring gear with 41 teeth and an 11-tooth pinion is 3.73 to 1. At 60 mph with that axle, a 1:1 transmission gear, and a 28 inch tire, the engine turns about 2,670 rpm.

Road speed, engine rpm, and tire diameter are one drivetrain. After this gear ratio result, ring and pinion ratio and transmission ratio and RPM should be read with the same tire and the same gear.

Mistakes that move the answer

• Overall ratio includes the transmission gear you are actually in, not just the axle.
• Tire diameter is the mounted overall diameter, not the wheel size printed on the sidewall.

How to check the result

After Calculate shows the gear ratio result, redo the last operation with a pencil using the rounded figures on the screen. Matching that check means the inputs were read correctly. If you edit a label or a sentence in this HTML file, reload the page. The form and this article both live in the file, so the edit is what visitors will read.