Overall ratio
For each gear, overall ratio equals transmission gear ratio x transfer case ratio x axle ratio.
Overall ratio = transmission ratio x transfer case ratio x axle ratioGear Ratio Calculator
Calculate theoretical vehicle speed or engine RPM for each gear using your tire size, transmission, transfer case, and axle ratios. Compare two setups when you need a side-by-side view.
Calculate RPM or Speed
Enter the values for your drivetrain. Results update when all required inputs are valid.
Enter the engine speed to use for every gear.
Check the highlighted fields
How This Gear Ratio Calculator Works
The calculator combines tire circumference with the transmission, transfer case, and axle ratios you enter. It keeps full precision during calculation and rounds only the displayed results.
Tire revolutions per mile
Using a nominal tire diameter in inches, tire revolutions per mile equals 63,360 divided by pi times the tire diameter.
Tire rev/mi = 63,360 / (pi x tire diameter in inches)Speed and engine RPM
The calculator uses the unrounded tire revolutions and overall ratio to solve for either theoretical speed or engine RPM.
Engine RPM = speed in mph x tire rev/mi x overall ratio / 60Speed in mph = engine RPM x 60 / (tire rev/mi x overall ratio)Metric tire size
For a metric size such as 285/70R17, sidewall height is tire width multiplied by aspect ratio, then converted from millimeters to inches. Two sidewalls are added to the wheel diameter. The result is a nominal geometric diameter, not a measured rolling diameter.
Sidewall height in inches = (width in mm x aspect ratio / 100) / 25.4Nominal tire diameter in inches = wheel diameter + (2 x sidewall height)Crawl ratio
First-gear crawl ratio equals first gear ratio x transfer case ratio x axle ratio. It describes the mathematical reduction in the values entered; it does not rate traction, safety, durability, fitment, or real-world crawl performance.
Crawl ratio = first gear ratio x transfer case ratio x axle ratioGear Ratio Calculator FAQ
What does this gear ratio calculator calculate?
It calculates theoretical vehicle speed at a selected engine RPM or theoretical engine RPM at a selected speed for each transmission gear. It also shows overall ratio, nominal tire diameter, tire revolutions per mile, and first-gear crawl ratio. When comparison is enabled, it shows the numerical difference between two configurations.
How do I calculate speed from RPM and gear ratio?
Enter engine RPM, nominal tire diameter, transmission gear ratio, transfer case ratio, and axle ratio. The calculator divides engine revolutions by the combined drivetrain reduction and converts tire revolutions into road speed. The formula used is: speed in mph = engine RPM x 60 / (tire revolutions per mile x overall ratio).
How do I calculate RPM at a given speed?
Enter the target speed and the same tire and drivetrain values. The formula is: engine RPM = speed in mph x tire revolutions per mile x overall ratio / 60. The result is theoretical and may not match a tachometer under real operating conditions.
What is overall gear ratio?
Overall ratio is the transmission gear ratio multiplied by the transfer case ratio and the axle or final drive ratio. For example, a 3.00:1 transmission gear, 1.00:1 transfer case, and 4.10:1 axle produce a 12.30:1 overall ratio.
What is crawl ratio?
Crawl ratio is the combined reduction of first gear, the selected transfer case ratio, and the axle ratio. A numerical crawl ratio does not by itself determine traction, component strength, vehicle suitability, or safe operating conditions.
How is a metric tire size converted to diameter?
For a size such as 285/70R17, the calculator converts the sidewall height from millimeters to inches, doubles it, and adds the wheel diameter. The result is a nominal geometric diameter, not an actual rolling diameter or loaded radius.
Why can actual RPM or speed differ from the result?
The calculator cannot measure actual rolling radius or account for every operating condition. Tire construction, pressure, deformation, wear, load, slip, torque-converter behavior, mechanical efficiency, component tolerances, and instrument or measurement error can all change real-world RPM or speed.