Gear Ratio Calculator
Work out your car's road speed at any engine RPM, or the RPM at any speed, from the gear ratio, final drive ratio and tire diameter. Results in mph and km/h.
Need your tire diameter? Work it out with the tire size calculator
Results
Road speed
54.42 mph
At the engine speed you entered, in theory, with no slip.
- Road speed in km/h
- 87.58 km/h
- Engine speed at target
- 3,308 rpm
- Overall drive ratio
- 4.1 : 1
Road speed 54.42 miles per hour, or 87.58 kilometres per hour. Engine speed at the target road speed 3,308 rpm.
Formula
- Overall drive ratio
- = gear ratio × final drive ratio
- Wheel RPM
- = engine RPM ÷ overall drive ratio
- Tire circumference (in)
- = π × tire diameter (in)
- Road speed (mph)
- = wheel RPM × circumference (in) × 60 ÷ 63,360
- Road speed (km/h)
- = road speed (mph) × 1.609344
- Engine RPM at a road speed
- = speed (mph) × 63,360 ÷ 60 ÷ circumference (in) × overall drive ratio
63,360 is the number of inches in a mile, and 60 turns revolutions per minute into revolutions per hour. The calculator works in millimetres and km/h internally, which gives exactly the same result because a mile is exactly 1,609,344 mm.
Worked example
A car in a 1.00 gear with a 4.10 final drive runs 25-inch tires. How fast is it going at 3,000 rpm, and what RPM does it need for 60 mph?
Step 1: Overall drive ratio
1.00 × 4.10 = 4.1 : 1
Step 2: Wheel RPM
3,000 ÷ 4.1 = 731.71
Step 3: Tire circumference
π × 25 = 78.54 in
Step 4: Road speed
731.71 × 78.54 × 60 ÷ 63,360 = 54.42 mph (87.58 km/h)
Step 5: Engine RPM at 60 mph
60 × 63,360 ÷ 60 ÷ 78.54 × 4.1 = 3,308 rpm
At 3,000 rpm the car travels at about 54.42 mph (87.58 km/h), and it needs about 3,308 rpm to hold 60 mph.
Where to find your ratios
Each gear's ratio and the final drive ratio are usually listed in the owner's manual, the manufacturer's technical specifications or a service manual. On many rear-wheel-drive vehicles the axle ratio is also on a tag on the differential or on the build sheet.
A gear ratio below 1.00, such as 0.70, is an overdrive: the output turns faster than the engine, lowering RPM at cruising speed. Use the ratio of the gear you want to check, together with the final drive for that gear.
How final drive and tire size change RPM
Engine RPM at a given speed is proportional to the final drive ratio. With the example car in the same gear, a 3.73 final drive needs about 3,009 rpm for 60 mph instead of 3,308 rpm with 4.10.
Bigger tires cover more ground per turn, so they lower RPM at a given speed in the same way. Moving the example car to 28-inch tires drops the RPM at 60 mph to about 2,953 rpm. If you have changed tire size, the tire size calculator gives the new diameter.
Why real speed and RPM can differ
The calculator gives theoretical gearing: the speed and RPM if the tire turned at exactly its nominal size with no slip. Real figures can differ because of:
- the tire's loaded rolling radius, which is smaller than its unloaded size;
- tread wear and tire pressure;
- tire growth at high speed;
- wheel slip under hard acceleration;
- torque-converter slip in automatic transmissions that is not locked up;
- speedometer and tachometer calibration.
Frequently asked questions
How do I calculate speed from RPM and gear ratio?
Divide the engine RPM by the gear ratio times the final drive ratio to get wheel RPM, multiply by the tire circumference in inches and by 60, then divide by 63,360. For 3,000 rpm, a 1.00 gear, a 4.10 final drive and 25-inch tires, that is about 54.42 mph.
How do I calculate RPM at a given speed?
Multiply the speed in mph by 63,360, divide by 60 and by the tire circumference in inches to get wheel RPM, then multiply by the overall drive ratio. The example car needs about 3,308 rpm for 60 mph.
What is a final drive ratio?
It is the reduction in the differential (or transaxle) between the gearbox output and the wheels. A 4.10 final drive turns the gearbox output 4.1 times for every turn of the wheels.
Can I enter tire size in millimetres or speed in km/h?
Yes. Choose mm or in for the tire diameter and mph or km/h for the target speed. Road speed is always shown in both mph and km/h.
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