Speedometer Error Calculator

What Speedometer Error is for

Speedometer Error Calculator is the page to open when you need speedometer error 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. Old tire width (mm). Enter the value this part of the job needs.
2. Old aspect ratio. Enter the value this part of the job needs.
3. Old wheel (in). Enter the value this part of the job needs.
4. New tire width (mm). Enter the value this part of the job needs.
5. New aspect ratio. Enter the value this part of the job needs.
6. New wheel (in). Enter the value this part of the job needs.
7. Speedometer. Enter the value this part of the job needs. The unit list on this field includes mph, km/h.

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 speedometer error result, speedometer gear and overall tire diameter 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 speedometer error 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.