Starlight Tools

Gear Ratio and RPM Calculator

Calculate theoretical engine RPM, road speed, transmission gear ratio, or final-drive ratio from tire size and the rest of the driveline. Inputs stay in this browser tab.

The result is a no-slip theoretical estimate Actual readings can differ because a tire’s loaded rolling circumference is not its nominal diameter, and because of torque-converter slip, clutch slip, tire growth, wheelspin, transfer-case gearing, or instrument error. Confirm component ratings and vehicle-specific limits independently.

Tire and driveline inputs

Tire size

Speed and gearing

Example: 0.70 for overdrive, 1.00 for direct.

Example: enter 3.73 for 3.73:1.

Privacy: input values are calculated locally. They are not uploaded, stored, or included in analytics events.

Theoretical driveline result

RPM, speed, and overall-ratio formulas

The calculator first finds nominal unloaded tire diameter from a metric sidewall size:

diameter (in) = rim diameter + 2 × (width in mm × aspect ratio ÷ 100 ÷ 25.4)

Transmission ratio and final-drive ratio multiply to give the overall reduction between engine and drive wheels:

overall ratio = transmission gear ratio × final-drive ratio

Using mph and diameter in inches, the common rounded relationship is:

engine RPM ≈ mph × overall ratio × 336 ÷ tire diameter

This tool uses the unrounded circumference relationship: wheel RPM = road inches per minute ÷ (π × diameter), then engine RPM = wheel RPM × overall ratio. The speed and ratio modes rearrange the same equation.

How to use the calculator

  1. Choose the value to solve. The calculated field is disabled so it cannot be mistaken for a required input.
  2. Describe the tire. Enter the three numbers from a metric size such as 275/60R15, or use a comparable measured or manufacturer-specified diameter.
  3. Enter the active gear. Use the ratio for the transmission gear being evaluated, not the transmission model’s first-gear ratio unless first gear is the intended case.
  4. Enter the final drive. Use the axle or differential ratio that applies at the driven wheel. Account separately for any engaged transfer-case, portal-axle, or auxiliary reduction.
  5. Review the theoretical result. Compare the primary value and speed-to-RPM table, then check real tire and vehicle data before choosing components or operating limits.

How tire size, gearing, and overdrive change RPM

  • A numerically higher final-drive ratio raises engine RPM by the same proportion when speed, tire diameter, and transmission gear stay constant.
  • A taller tire lowers theoretical RPM because each wheel revolution covers more distance.
  • An overdrive gear has a ratio below 1.00, so engine RPM is lower than it would be in a 1.00:1 direct gear at the same road speed.
  • A ratio result describes the selected operating point only. It does not establish suitable launch performance, gradeability, shift spacing, fuel economy, durability, or maximum speed.

Nominal versus rolling size: sidewall math estimates an unloaded diameter. Published revolutions-per-mile data or a rolling measurement under the relevant load can produce a closer road-speed estimate.

Gear ratio and RPM FAQs

How do I calculate RPM from speed and gear ratio?

Find wheel RPM from road speed and tire circumference, then multiply by the transmission gear ratio and final-drive ratio. In US customary units, the familiar rounded form is RPM ≈ mph × transmission ratio × final-drive ratio × 336 ÷ tire diameter in inches.

What does the 336 constant mean?

It combines 63,360 inches per mile, 60 minutes per hour, and π. The exact constant is approximately 336.135 when a perfectly circular diameter is assumed; the calculator uses the underlying circumference calculation rather than rounding to 336.

Does a higher final-drive ratio increase RPM?

Yes. Changing from 3.55:1 to 4.10:1 increases theoretical engine RPM at the same speed, tire diameter, and transmission gear by a factor of 4.10 ÷ 3.55.

Should I enter overdrive ratio or overall ratio?

Enter only the active transmission gear ratio in the transmission field and the axle or differential ratio in the final-drive field. The calculator multiplies them and reports the overall ratio.

Why does my tachometer show a different RPM?

Real rolling circumference can differ from nominal diameter. An unlocked torque converter, clutch slip, wheelspin, tire growth, another reduction stage, tachometer error, or speedometer error can also create a difference.

Can this calculator select a safe axle ratio?

No. It models rotational speed only. A component choice also requires verified engine speed limits, transmission and axle capacity, tire ratings, cooling, traction, noise, emissions, fitment, and legal requirements.

Are my speed and gearing inputs tracked?

No. The calculation runs locally in your browser. This tool does not upload, store, or attach input values to analytics events.

Limits and safety disclaimer

  • The model assumes a direct mechanical relationship with no slip and a circular tire of constant effective diameter. It does not simulate an unlocked torque converter, CVT behavior, clutch slip, tire deformation, or wheelspin.
  • Only one transmission ratio and one final-drive ratio are included. Multiply any engaged transfer-case, portal, hub, or auxiliary ratio into the appropriate known reduction before calculating.
  • The result does not assess engine redline, driveshaft critical speed, tire speed rating, component strength, braking, handling, emissions, fuel use, noise, fitment, or road legality.
  • Do not take measurements or interact with this tool while driving. Use published specifications or have a passenger or qualified professional collect data safely.

Safety disclaimer: This educational estimate is not a component recommendation or substitute for manufacturer service information and professional engineering judgment. Verify the complete driveline and all operating limits before modifying or operating a vehicle.

Methodology and sources

Last reviewed: July 31, 2026. The variables, tire-size interpretation, and theoretical driveline approach were checked against manufacturer and government material:

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