Gear Ratio, Tire Size & RPM Calculator
Compare road speed and engine RPM before you choose your gearing.
Calculated results
Find your cruising range.
| Gear | Ratio | Overall | MPH at 2500 RPM | RPM at 65 MPH |
|---|
Chart: road speed in MPH, engine speed in RPM. Dashed line marks your selected redline.
How to calculate engine RPM and road speed
- Select a transmission preset, or choose custom ratios and enter each forward gear separated by commas. Verify the ratios for your transmission version.
- Enter your rear axle ratio and tire diameter in inches. Alternatively, select metric tire size and enter a size such as 225/70R15.
- Set reference RPM and reference speed. Keep both filled: the table calculates MPH at that RPM and RPM at that speed for every gear.
- Set the engine redline and compare the table and chart. The dashed line is your selected RPM limit; it is not a recommended limit for your engine.
Gear ratio and tire size formulas
Overall ratio = transmission gear ratio × rear axle ratio
RPM = MPH × overall ratio × 336 ÷ tire diameter (inches)
MPH = RPM × tire diameter (inches) ÷ (overall ratio × 336)
The factor 336 is a rounded conversion from tire circumference, inches per mile and minutes per hour. These calculations assume no clutch, tire or torque-converter slip.
Example: highway RPM with 28-inch tires
At 65 MPH, a 3.54 rear axle, 1.00:1 top gear and 28-inch tire give approximately 2,761 RPM. Changing only top gear to a 0.68 overdrive gives approximately 1,878 RPM. Changing only the axle from 3.54 to 4.11 raises direct-drive cruise RPM to approximately 3,206 RPM.
A numerically higher axle ratio increases engine RPM at a given speed. A larger tire lowers it. Lower cruising RPM is not automatically better: the engine still needs enough torque at that speed to pull the car comfortably.
Reading tire sizes
For a metric tire, nominal diameter in inches equals wheel diameter plus twice the sidewall height. Sidewall height is section width in millimeters multiplied by the aspect ratio as a decimal, then divided by 25.4. A 225/70R15 is approximately 27.40 inches by this calculation. Manufacturer dimensions and actual rolling circumference can differ.
Gearing questions
Does the tire size field mean wheel diameter?
No. Enter the tire's overall diameter, including both sidewalls. A 15-inch wheel may carry a tire roughly 27–30 inches tall, depending on the size.
Which transmission ratios are included?
Presets include the Ford 1939-style three-speed, close-ratio Ford Toploader four-speed, T-85, Muncie M20 and M21/M22, early T10 and two T5 combinations. These are specific example ratio sets, not a guarantee for every transmission carrying that name.
Why is my tachometer reading different?
Actual rolling tire diameter, converter or clutch slip, an incorrect axle ratio, and tachometer or speedometer error can all change the observed result. An unlocked torque converter commonly makes engine RPM higher than a no-slip estimate.
Does the chart predict top speed?
It shows the gearing relationship between speed and RPM. It does not account for aerodynamic drag or whether the engine has enough power to reach a particular speed.
Related calculators and workshop references
Return to all automotive calculators, check original specifications in the automotive manual library, or browse hot rod parts and workshop tools.