Calculated Tire Diameter
0 in
Automotive Calculator
Gear Ratio and RPM Calculator
Calculate engine RPM at a selected road speed or estimate vehicle speed at a selected RPM using tire diameter, transmission ratio, and axle ratio.
Drivetrain Estimate
Estimated engine speed
0 RPM
Overall Gear Ratio
0:1
Tire Revolutions per Mile
0
Speed per 1,000 RPM
0 MPH
Calculation Summary
How gear ratios affect engine RPM
A numerically higher axle ratio, such as 4.56, turns the engine more times for each tire revolution than a numerically lower ratio, such as 3.08. This usually increases engine RPM at a given road speed.
A transmission overdrive ratio below 1.00 lowers cruising RPM. A 1.00 ratio is direct drive, while a ratio such as 0.70 reduces engine speed in the selected gear.
How tire size changes RPM
A taller tire travels farther with each revolution. With the same transmission and axle ratios, increasing tire diameter lowers engine RPM at a given road speed. A shorter tire raises RPM.
Metric tire size is written as width, aspect ratio, and wheel diameter. For example, a 265/70R17 tire is approximately 31.6 inches tall before accounting for manufacturer differences, tire wear, inflation, and vehicle load.
Common uses
- Compare axle ratios before a differential gear change
- Estimate highway RPM after changing tire size
- Compare transmission gears or overdrive ratios
- Plan engine, transmission, or axle swaps
- Estimate theoretical road speed at a chosen engine RPM
Calculation formulas
RPM = MPH × transmission ratio × axle ratio × 336 ÷ tire diameter
MPH = RPM × tire diameter ÷ (transmission ratio × axle ratio × 336)