Calculator Inputs
Example Data Table
| Scenario | Tire Size | Gear Ratio | Axle Ratio | Engine RPM | Tire Diameter (in) | Overall Ratio | Estimated Speed (mph) |
|---|---|---|---|---|---|---|---|
| Highway Overdrive Setup | 265/70R17 | 0.78 | 3.73 | 2,400 | 31.61 | 2.91 | 75.24 |
| Towing Mid Gear Setup | 285/75R16 | 1.00 | 4.10 | 2,800 | 32.83 | 4.10 | 65.37 |
| Low Range Crawl Setup | 315/70R17 | 2.84 | 4.88 | 1,800 | 34.36 | 37.70 | 4.83 |
| Street Performance Setup | 245/45R18 | 0.67 | 3.55 | 3,000 | 26.68 | 2.38 | 98.11 |
Formula Used
Sidewall Height = (Section Width × Aspect Ratio ÷ 100) ÷ 25.4
Tire Diameter = Wheel Diameter + 2 × Sidewall Height
Circumference = π × Tire Diameter
Overall Ratio = Transmission Gear Ratio × Axle Ratio × Transfer Case Ratio
Wheel RPM = Effective Engine RPM ÷ Overall Ratio
Effective Engine RPM = Engine RPM × (1 − Slip % ÷ 100)
Speed = (Wheel RPM × Circumference × 60) ÷ 63,360
Required RPM = [Target Speed × 63,360 ÷ (Circumference × 60)] × Overall Ratio ÷ Slip Factor
These formulas convert tire geometry into rolling distance, then connect wheel speed to engine speed through the combined drivetrain ratio. The result helps compare cruise RPM, towing behavior, and low-range crawl performance using consistent mechanical relationships.
How to Use This Calculator
- Select whether you want to enter a full tire size code or a direct tire diameter.
- Enter the transmission gear ratio, axle ratio, and transfer case ratio for the exact gear you want to analyze.
- Provide engine RPM to estimate road speed for that setup.
- Enter a target road speed to estimate the RPM needed to hold that speed.
- Add a small slip percentage when you want a more realistic automatic or driveline estimate.
- Submit the form and review the result block above the form.
- Use the Plotly graph to compare speed across a larger RPM range.
- Export the current result summary using the CSV or PDF buttons.
Frequently Asked Questions
1) What does this calculator actually solve?
It links tire size, transmission gear, axle ratio, transfer case ratio, and engine RPM. The output estimates road speed, wheel RPM, revolutions per mile, and the RPM required for a chosen speed.
2) Why does tire diameter matter so much?
A larger tire travels farther per revolution. That increases vehicle speed at the same wheel RPM, lowers cruise RPM, and changes how your final gearing feels on the road.
3) Should I use direct gear or overdrive?
Use the actual transmission ratio for the gear you want to inspect. Direct drive is often near 1.00. Overdrive gears are below 1.00 and usually lower highway RPM.
4) What is the purpose of transfer case ratio?
It lets the calculator handle both high range and low range. Use 1.00 for high range. Use the low-range multiplier when analyzing crawling or off-road gearing.
5) Why include slip percentage?
Real drivetrains are not perfectly locked all the time. A small slip percentage can model torque converter slip or minor drivetrain losses, making the speed estimate more realistic.
6) Is this useful for tire size swaps?
Yes. Change only the tire size and keep the same gearing. The output quickly shows how much your cruise speed, RPM, and effective gearing shift after the tire upgrade.
7) Can I compare crawl ratio with highway behavior?
Yes. Enter a low transmission gear and low-range transfer ratio to inspect off-road multiplication. Then switch to overdrive and high range to review highway RPM and speed.
8) Are these results exact for every vehicle?
They are strong engineering estimates, but real speed can vary with true loaded tire diameter, converter behavior, tire growth, wear, pressure, and drivetrain efficiency.