Engine RPM at Speed Calculator
Calculate engine RPM at any road speed from tire size, transmission gear ratio and axle ratio. Works in mph and km/h.
Combine tire size and drivetrain ratios.
Engine RPM at Speed Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Engine RPM at Speed Calculator Works?
The Engine RPM at Speed Calculator works by converting road speed into wheel revolutions per minute and multiplying by the transmission and axle ratios. The tool uses 4 steps.
Enter the tire
Type the tire size or diameter, for example 31.6 in (803 mm).
Enter the ratios
Type the gear ratio of the selected gear (0.70 for an overdrive) and the axle ratio (3.73).
Enter the speed
Type the road speed, for example 65 mph (105 km/h).
Read the RPM
The calculator returns about 1,805 engine RPM, plus the RPM with torque converter slip if a slip percent is entered.
Engine RPM at Speed Calculator Formula
The Engine RPM at Speed Calculator formula is RPM = (MPH × Gear × Axle × 336.13) ÷ Tire diameter (in).
RPM = MPH × G × A × 336.13 ÷ D_inRPM = km/h × G × A × 5,305.2 ÷ D_mmRPM_wheel = MPH × 336.13 ÷ D_in- G
- Transmission gear ratio
- A
- Axle (final drive) ratio
- 336.13
- 63,360 ÷ (60 × π)
- 5,305.2
- 1,000,000 ÷ (60 × π)
Worked example
A truck in a 0.70 overdrive with 3.73 gears and 33 in tires at 70 mph runs 70 × 0.70 × 3.73 × 336.13 ÷ 33 = 1,862 RPM. On stock 31.6 in tires, the same speed gives 1,944 RPM.
Engine RPM at Speed Calculator Conversion Table
The Engine RPM at Speed Calculator conversion table lists engine RPM at 4 speeds for 6 tire diameters with a 0.70 overdrive and 3.73 axle.
| Tire diameter | 45 mph (72 km/h) | 55 mph (89 km/h) | 65 mph (105 km/h) | 75 mph (121 km/h) |
|---|---|---|---|---|
| 26 in (660 mm) | 1,519 | 1,857 | 2,194 | 2,532 |
| 28 in (711 mm) | 1,411 | 1,724 | 2,037 | 2,351 |
| 30 in (762 mm) | 1,316 | 1,609 | 1,902 | 2,194 |
| 31.6 in (803 mm) | 1,250 | 1,528 | 1,805 | 2,083 |
| 33 in (838 mm) | 1,197 | 1,463 | 1,729 | 1,995 |
| 35 in (889 mm) | 1,128 | 1,379 | 1,630 | 1,881 |
What is Engine RPM at Speed Calculator?
An Engine RPM at Speed Calculator is a tool that predicts engine speed at a given road speed from tire size, gear ratio and axle ratio. The calculation assumes a locked torque converter or a manual clutch with no slip.
Tire size changes RPM in inverse proportion. A 4% taller tire lowers cruise RPM by about 4%.
Cruise RPM shapes fuel use, cabin noise and whether the transmission holds top gear on hills.
What are the uses of Engine RPM at Speed Calculator?
There are 4 main uses of the Engine RPM at Speed Calculator, including highway cruise checks and gear swap planning.
Highway cruise checks
Find cruise RPM before and after a tire change.
Gear swap planning
Test axle ratios against a target RPM.
Engine swap planning
Match a new engine's torque peak with cruise speed.
Overdrive decisions
Compare RPM with and without an overdrive gear.
Related Speedometer & Distance Tools
View All 134 Subtools →Estimate speed changes after tire resizing.
Estimate road speed from indicated speed.
Estimate displayed speed at a given road speed.
Calculate a tire-size correction multiplier.
Tabulate indicated and estimated actual speeds.
Estimate distance recording differences.