The Gear-Tire-Speed Reference Tools

cribbed, not authored, by yours truly

This program is for calculating the speed (MPH) for a given transmission gear and differential. All calculations are round up. Calculations are to be treated as approximate only.

To use this program you need to key in five (5) pieces of information. The tire size of the driven wheel (tread width, sidewall ratio, and wheel rim diameter) transmission gear ratio and differential ratio (final drive). This information can then be used to compare different tire and wheel combinations to tranmission gears and differentials.

The following is an example to help to decode a tire size like 185/65R14
185 = tire width in millimeters, measured from sidewall to sidewall
65 = sidewall ratio, the ratio of sidewall height to tire width
14 = wheel rim diameter in inches

The transmission gear ratio needs to be expresses in a decimal formate, like 1.444.

The differential ratio needs to be expressed in a decimal formate, like 3.444. If you have the number expressed as a ring and pinion like 7:31, you need to calculate the ratio by dividing the second number (31) by the first (7) to come up with the correct format and in this case it would be 4.429.

After keying in the five (5) pieces of information, click on the "Show Speed" button below to start the calulations.

Gear Speed Data Entry Section

Step 1: Key in the following information.
Original Equipment Tire
Tire Width in (mm):
Aspect Ratio:
Rim Diameter in (inch):
Transmission Gear Ratio:
Differential Ratio (final drive):

Step 2: To calculate the speed, please click on the "Show Speed" button below.

Step 3: To see the results of the calculations SEE BELOW:

Gear Speed Information Section
Engine Speed (RPM) Speed (MPH)

To calculate or compare different gears and transmissions, go back to the Gear Speed Data Entry Section and type over the information you want to change. After making the changes, click on the "Show Speed" button to recalculate.

To reset this form, click on "Clear Form" button below.