How to use the gear ratio calculator
- Transmission gear ratio: the ratio of the gear you are driving in, from your owner's manual or spec sheet. A top gear of 0.75 is a typical overdrive.
- Axle ratio (final drive): for example 3.73. If you only know tooth counts, fill in Driven gear teeth (ring gear) and Drive gear teeth (pinion), then press the Use button.
- Tire diameter (in): the overall height of the tire. The tire size calculator works it out from a size such as 225/65R17.
- Speed and Speed unit: the road speed you want to check.
- Optional: Compare with axle ratio shows a second axle side by side, and Redline or target RPM gives the speed you reach at that engine speed.
How the numbers are worked out
Engine RPM = speed in mph × 1,056 × transmission ratio × axle ratio ÷ (π × tire diameter in inches). The 1,056 is the 63,360 inches in a mile divided by 60 minutes. Dividing by the tire circumference gives tire turns per minute, and the gear ratios turn those into engine turns. The 336 shortcut found on many sites is 1,056 ÷ π = 336.1 rounded. A tooth ratio is driven gear teeth ÷ drive gear teeth, as defined in Wikipedia, Gear train.
Worked example with the starting numbers
A 0.75 gear and a 3.73 axle give an overall ratio of 2.7975, shown as 2.80:1. With 28.5 in tires at 70 mph, 70 × 1,056 × 2.7975 ÷ (π × 28.5) = 2,310 RPM. The table shows 990 RPM at 30 mph and 2,640 RPM at 80 mph. At a 6,000 RPM redline this gear reaches 181.8 mph. Entering 4.10 under Compare with axle ratio gives 2,539 RPM at 70 mph (+9.9%), and 31.33 in tires would bring it back to 2,310.
Limits and edge cases
- An automatic's torque converter slips when it is not locked, so the engine can turn faster than the formula says. In a gear where the converter is locked the figures hold.
- A loaded or worn tire rolls slightly smaller than its listed diameter, so real RPM can run a little higher.
- Speed here is true road speed, not the speedometer reading, which bigger tires make read low.
- Inputs must be positive and within sensible ranges, such as 10 to 60 in for tire diameter. A message names the range when a value is out.
- To see how much power the engine makes at an RPM, use the horsepower and torque calculator. For other units, try the speed converter.
Gear ratio questions
How do I calculate engine RPM from speed, gear ratio and tire size?
Multiply the speed in mph by 1,056, the transmission ratio and the axle ratio, then divide by π times the tire diameter in inches. At 70 mph in a 0.75 gear with a 3.73 axle and 28.5 in tires, that is 70 × 1,056 × 0.75 × 3.73 ÷ (π × 28.5) = 2,310 RPM.
Is a higher or lower axle ratio better?
A numerically higher ratio such as 4.10 multiplies engine torque more, so the vehicle accelerates and tows more easily, but the engine turns faster at every speed. Here a 4.10 axle raises RPM at 70 mph by 9.9% compared with 3.73. A numerically lower ratio does the opposite. Neither is best for every vehicle.
How do I find my axle ratio?
Check the owner's manual, the build sheet or the tag on the axle. If the differential is open you can count the teeth on the ring gear and the pinion and divide: 41 ÷ 11 = 3.73.
How much do bigger tires change engine RPM?
RPM falls by about the same percentage that the tire diameter grows. Tires 3% taller lower engine speed by about 2.9% at the same road speed. Work out the new diameter with the tire size calculator first, then enter it here.
Source: Wikipedia, Gear train (gear ratio from tooth counts). Engine speeds are calculated from nominal sizes and ratios, not measured.