How to use the gear ratio calculator
- Choose what to find: RPM at a speed, speed at an RPM or ratio from gear teeth.
- For RPM or speed, enter the axle ratio, the transmission gear ratio (1 for a direct gear) and the tire diameter in inches.
- Enter the speed in mph, or the engine speed in rpm, depending on what you are finding.
- For gear teeth, enter the ring gear and pinion gear tooth counts. Read the result and the table of other speeds.
Formula
Total ratio = axle ratio x transmission gear ratio
Tire circumference = pi x tire diameter
Wheel rpm = speed in mph x 63,360 / 60 / circumference
Engine rpm = wheel rpm x total ratio
Axle ratio from teeth = ring gear teeth / pinion gear teeth
One mile is 63,360 inches. Speed from rpm reverses the same steps.
A 3.73 axle with 28-inch tires at 60 mph
The tire circumference is pi x 28 = 87.96 inches, so at 60 mph the wheels turn 60 x 63,360 / 60 / 87.96 = 720.3 times a minute. In a direct gear (ratio 1.00) the engine turns 720.3 x 3.73 = 2,687 rpm. At 40 mph the same setup gives 1,791 rpm and at 80 mph 3,582 rpm.
Going the other way, 2,500 rpm in the same setup is about 55.8 mph. A ring gear with 41 teeth and a pinion with 11 teeth gives 41 / 11 = 3.727, which is usually quoted as 3.73.
What each ratio does
A higher axle ratio, such as 4.10, turns the engine faster at the same road speed and gives more pulling power at the cost of higher rpm on the highway. A lower number does the reverse. An overdrive gear below 1 lowers engine speed further.
The transmission ratio is the gear you are in, so to see cruising rpm enter the highest gear, and to see engine speed at launch enter a low one.
Accuracy and limits
The tire diameter you enter should be the rolling diameter. A loaded tire is a little smaller than its unloaded height. Torque converter slip in an automatic means real rpm can be higher until the converter locks.
To work out a tire's overall diameter from its size code use the tire size calculator. The speedometer error calculator shows how a different tire changes your reading, and the horsepower from torque calculator links engine speed to power.
Assumptions and limits
- Tire diameter is in inches and is the rolling diameter.
- There is no tire slip and no torque converter slip.
- The gear ratios are those of your own vehicle and the values shown are only examples.
- Transfer cases, differentials with other ratios and wheel speed sensors are not modeled.
Frequently asked questions
How do you calculate rpm from speed?
Find how many times the tire turns per minute from the speed and its circumference, then multiply by the axle ratio and the transmission gear ratio.
How do you calculate the axle ratio from gear teeth?
Divide the number of teeth on the ring gear by the number on the pinion. A ring gear with 41 teeth and a pinion with 11 gives about 3.73.
What gear ratio should I use for the transmission?
Use the ratio for the gear you are in, from your vehicle's specifications. Enter 1 for a direct drive gear, and a number below 1 for an overdrive gear.
Why might my tachometer read differently?
Tire wear, tire pressure, load and torque converter slip change the real numbers. This tool also assumes the tire diameter you enter is accurate.