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RPM to Gear Ratio Calculator
Your setup
Tire diameter is the overall height of the tire in inches. The formula uses the standard constant 336, which converts miles per hour and tire circumference into revolutions per minute.
Example: 65 mph at 3,000 RPM on 28 in tires → a 3.85 gear ratio. With 3.73 gears at the same speed and tire size, the engine turns about 2,909 RPM.
Behind the numbers
How gear ratio is calculated from RPM
Engine RPM, road speed and tire size are linked by one formula. The tire rolls forward one circumference per revolution, and the constant 336 converts miles per hour and inches into revolutions per minute. Solve the same formula for RPM when you already know the gear ratio.
RPM = (MPH × gear ratio × 336) ÷ tire diameter. Worked example: 3,000 × 28 = 84,000; 65 × 336 = 21,840; 84,000 ÷ 21,840 = 3.85. The same truck with 3.73 gears at 65 mph on 28 in tires turns 65 × 3.73 × 336 ÷ 28 = 2,909 RPM.
Common factory axle ratios
A higher number means more torque at the wheels and higher RPM at the same speed; a lower number means lower RPM and better highway fuel economy.
Examples
Example gear ratios
| Speed | Engine RPM | Tire diameter | Gear ratio |
|---|---|---|---|
| 65 mph | 3,000 | 28 in | 3.85 |
| 70 mph | 2,500 | 30 in | 3.19 |
| 55 mph | 3,200 | 26 in | 4.50 |
| 60 mph | 2,200 | 31 in | 3.38 |
| 65 mph | 2,909 (3.73 gears) | 28 in | 3.73 |
Tips before you buy
Measure the tire diameter across the tire while it is on the vehicle, or work it out from the sidewall size — a 275/55R20 is about 31.9 in tall.
A ratio near 3.73 is a common factory choice for trucks that tow; check your door sticker or differential tag to confirm the ratio you actually have.
Bigger tires lower your effective ratio: the same 3.73 gears feel taller (lower RPM) once you fit larger tires.
Use RPM at a steady cruising speed, not while accelerating — the formula assumes the transmission is in its top gear with the torque converter locked.
Good to know
Frequently asked questions
Where does the number 336 come from?
It is a unit-conversion constant. It combines the number of inches in a mile (63,360), the minutes in an hour (60) and pi for the tire circumference: 63,360 divided by (60 times pi) is about 336.13, rounded to 336 for this standard formula.
What gear ratio is best for towing?
Higher ratios such as 3.73 or 4.10 give more pulling power and keep the engine in its torque band, at the cost of higher highway RPM. Lower ratios such as 3.08 or 3.42 cruise at lower RPM but work harder with a heavy trailer.
How do I find my tire diameter from the sidewall?
Take the size printed on the sidewall, for example 275/55R20: the sidewall height is 275 mm times 55%, or 151.25 mm. Double it (302.5 mm, about 11.91 in) and add the 20 in wheel to get about 31.9 in overall.
Does this work for any car or truck?
Yes, for any vehicle with round tires and a fixed final drive ratio. It assumes the transmission is in direct drive (1:1 top gear) or that you enter the RPM already in that gear; for overdrive gears, divide the result by the overdrive ratio.