Calculator Inputs
Example Data Table
| Setup | RPM | Gear | Axle | Tire | Estimated MPH |
|---|---|---|---|---|---|
| Street cruiser | 3000 | 1.00 | 3.73 | 28 in | 66.94 |
| Overdrive highway | 2200 | 0.70 | 3.55 | 27 in | 71.12 |
| Short gear track | 6500 | 1.34 | 4.10 | 26 in | 91.58 |
Formula Used
Overall Ratio = Transmission Gear Ratio × Axle Ratio × Transfer Case Ratio
MPH = Engine RPM × Tire Diameter ÷ Overall Ratio ÷ 336.135
Slip Adjusted MPH = MPH × ((100 − Slip Percent) ÷ 100)
Metric tire diameter is calculated as: Wheel Diameter + 2 × ((Tire Width × Aspect Ratio ÷ 100) ÷ 25.4)
How to Use This Calculator
- Enter the engine rpm for the speed point you want to check.
- Add the active transmission gear ratio.
- Enter the axle or final drive ratio.
- Use 1.00 for transfer ratio unless a transfer case is engaged.
- Enter direct tire diameter, or choose metric tire size.
- Add estimated converter or clutch slip when needed.
- Press the calculate button to view speed above the form.
- Use the CSV or PDF button to save the result.
Engineering Guide
Why Gear Ratio Matters
Gear ratio controls how engine rotation becomes wheel rotation. A higher numerical ratio gives stronger multiplication. It can improve launch force. It can also reduce road speed at the same engine rpm. A lower numerical ratio works the opposite way. It often supports calmer highway travel. It may reduce acceleration when torque is limited.
Tire Diameter Changes Speed
Tire diameter acts like a rolling lever. A taller tire travels farther during each wheel turn. That raises speed for the same rpm. A shorter tire travels less distance. That lowers speed but can improve response. Real tires may measure differently under load. Wear, pressure, and tread style can change the effective value.
Useful Design Checks
This calculator helps compare axle swaps, overdrive gears, tire changes, and cruising rpm. It also estimates required rpm for a target speed. Use it before buying gears or tires. Small changes can create large differences. The overall ratio is especially important. It combines gearbox, axle, and transfer case values. Always compare the full system, not one part alone.
Practical Accuracy Notes
The formula uses tire diameter and a standard conversion constant. It gives a strong planning estimate. Actual speed can vary because of tire growth, converter slip, clutch slip, speedometer calibration, and measurement error. For race work, verify results with data logs. For street work, confirm readings with a reliable speed source. Use this tool as a drivetrain planning guide.
FAQs
What does a gear ratio to mph calculator do?
It estimates vehicle road speed from engine rpm, tire diameter, transmission ratio, axle ratio, and transfer ratio. It helps compare drivetrain combinations before changing gears, tires, or cruising rpm.
Why is tire diameter important?
Tire diameter decides how far the vehicle moves per wheel revolution. A taller tire increases speed at the same rpm. A shorter tire lowers speed but can improve acceleration feel.
What is overall gear ratio?
Overall gear ratio is the transmission gear ratio multiplied by axle ratio and transfer ratio. It shows the total reduction between engine speed and wheel speed.
Can I use metric tire size?
Yes. Choose the metric tire option, then enter width, aspect ratio, and wheel diameter. The calculator converts those values into an estimated tire diameter.
What should I enter for transfer ratio?
Use 1.00 for normal two wheel drive or high range. Enter the actual transfer case ratio when a reduction range is engaged.
What does slip percent mean?
Slip percent represents lost speed from converter or clutch slip. A value of zero means no adjustment. Higher values reduce the final road speed estimate.
Is the result exact?
The result is an engineering estimate. Real speed may change because of tire pressure, tire growth, tread wear, converter behavior, and speedometer calibration.
Can this help choose axle gears?
Yes. Compare several axle ratios at the same rpm and tire size. This helps balance acceleration, cruising rpm, fuel use, and top speed planning.