The formula (with the 336.13 constant explained)
speed (mph) = RPM × tire diameter (in) ÷ (gear ratio × 336.13)
speed (km/h) = mph × 1.60934
gear ratio (total) = transmission ratio × axle ratio
Where 336.13 comes from
One mile = 63,360 inches. In one hour the tire covers RPM × π × diameter inches per minute, × 60 minutes. Divide by inches-per-mile: 63,360 ÷ (60 × π) = 336.13. The radius-based version of the formula (v = r × RPM ÷ (168 × G)) uses half of it: 336.13 ÷ 2 ≈ 168.
Worked example
2,778 RPM, 3.73 gears, 31.6-inch tires:
2,778 × 31.6 ÷ (3.73 × 336.13) = 87,784.8 ÷ 1,253.77 = 70.0 mph (112.7 km/h).
Overdrive example: 0.70 × 3.73 = 2.61 total. At 2,500 RPM on 33-inch tires: 2,500 × 33 ÷ (2.61 × 336.13) = 94.0 mph.
How to use this calculator
- Enter your engine RPM (e.g. 2,778 at cruise).
- Enter the total gear ratio — or the transmission gear and axle ratio separately (e.g. 0.70 × 3.73).
- Enter tire diameter directly, or from a metric size (265/70R17) or flotation size (35x12.50R17).
- Read speed in mph and km/h, plus wheel RPM and tire revolutions per mile.
- Use the gear table to see speed in every gear and the gearing-limited top speed per gear.
Frequently asked questions
How do you calculate vehicle speed from RPM and gear ratio?
Use speed (mph) = RPM × tire diameter (inches) ÷ (gear ratio × 336.13). The gear ratio is the total reduction: transmission gear ratio multiplied by the axle (differential) ratio. Multiply mph by 1.60934 to get km/h.
What is the 336.13 constant in the gear ratio speed formula?
336.13 converts inches and revolutions per minute into miles per hour. It equals 63,360 inches per mile divided by (60 minutes per hour × π). The equivalent radius-based formula uses 168, which is 336.13 divided by 2.
What is the difference between gear ratio, final drive ratio, and total gear ratio?
The final drive (axle or differential) ratio is just the ring-and-pinion reduction, e.g. 3.73. The transmission gear ratio is the current gear, e.g. 0.70 overdrive. The total gear ratio multiplies both: 0.70 × 3.73 = 2.611. Always use the total ratio in the speed formula.
How does tire size affect speed at the same RPM?
Speed is directly proportional to tire diameter: a larger tire covers more ground per revolution, so the vehicle goes faster at the same engine RPM and gear ratio. Bigger tires also make the speedometer read low and raise the effective gearing.
How do I convert a metric tire size like 265/70R17 to diameter in inches?
Diameter (inches) = rim diameter + 2 × width (mm) × aspect ratio ÷ 100 ÷ 25.4. For 265/70R17: 17 + 2 × 265 × 0.70 ÷ 25.4 = 31.61 inches. This calculator does the conversion for you.
What gear ratio do I need to cruise 70 mph at 2,500 RPM?
Rearrange the formula: ratio = RPM × tire diameter ÷ (mph × 336.13). With 31.6-inch tires: 2500 × 31.6 ÷ (70 × 336.13) = 3.36 total ratio. Divide by your transmission gear (e.g. 0.70 overdrive) to get the axle ratio: 3.36 ÷ 0.70 = 4.80, so you would pick the nearest available axle such as 4.56 or 4.88.
How fast will my truck go with 4.10 gears and 35-inch tires?
In 0.75 overdrive with a 4.10 axle, the total ratio is 3.075. At 2,000 RPM with 35-inch tires: 2000 × 35 ÷ (3.075 × 336.13) = 67.7 mph (109 km/h). Enter your own RPM, gear, and tire size in the calculator for an exact number.
Why is my speedometer wrong after installing bigger tires?
The speedometer is calibrated to the stock tire diameter. Bigger tires travel farther per revolution, so true speed is higher than indicated: true speed = indicated speed × new diameter ÷ old diameter. The taller tire also lowers your effective gear ratio.
How do I calculate my speed in each gear of the transmission?
Run the speed formula once per gear: multiply each transmission gear ratio by the axle ratio to get that gear's total ratio, then compute speed at your RPM. The gear table mode on this page does this automatically and also shows the top speed in each gear at redline.
What is the difference between engine RPM and wheel RPM?
Engine RPM is crankshaft speed; wheel RPM is engine RPM divided by the total gear ratio. At 2,778 RPM with a 3.73 total ratio, the wheels turn 745 times per minute. Wheel RPM times tire circumference gives road speed.
Can I use this gear ratio speed calculator for motorcycles?
Yes. Motorcycles, cars, trucks, and any vehicle with a gearbox and round tires follow the same formula. Use the primary reduction × gearbox × final drive as the total ratio, and the rear tire diameter in inches.
3.73 vs 4.10 gears: which is faster on the highway?
At the same RPM and tire size, 3.73 gears give about 10% more speed than 4.10 gears (speed is inversely proportional to ratio: 4.10 ÷ 3.73 = 1.10). Conversely, 4.10 gears turn about 10% more RPM at the same speed, which helps towing and acceleration but hurts cruising fuel economy.
Is the theoretical top speed from gearing achievable in real life?
Not necessarily. The formula gives the gearing-limited speed assuming the engine can reach that RPM in that gear. In reality, aerodynamic drag, power, rev limiters, and speed governors usually cap top speed well below the gearing limit, especially in tall overdrive gears.
How is this different from a gear ratio RPM calculator?
This tool solves in the RPM-to-speed direction: given RPM, gearing, and tire size, it outputs vehicle speed. The companion gear ratio RPM calculator solves the inverse direction: given a target speed, it outputs the engine RPM. A third tool, the gear ratio calculator, solves for the ratio itself.
What does overdrive (e.g. 0.70) do to my speed?
An overdrive gear has a ratio below 1.0, which lowers the total gear ratio and raises speed at a given RPM (or lowers RPM at a given speed). Example: 0.70 overdrive with 3.73 axle = 2.611 total ratio, giving about 43% more speed at the same RPM than a 1:1 direct-drive gear.