This tool helps you estimate your vehicle’s road speed at a given RPM — or find the ideal rear-end gear ratio for your setup. Follow the steps below depending on what you want to calculate.
1. Enter Your Tire Diameter
- Measure your rear tire’s overall diameter (not just the wheel size).
- You can find this on the tire sidewall (example: 235/75R15 ≈ 28.9").
- Or measure from the ground to the top of the tire.
- Enter the diameter in inches.
2. Select Your Transmission Gear Ratio
-Select one of our preset transmissions that matches your setup, or enter your ratios manually.
-If you’re unsure, check your transmission specs online or in your manual.
3. Enter Your Rear Axle Ratio
-Input your rear differential ratio (e.g., 3.00, 3.25, 3.55, 4.11, 4.56, etc.).
(This is the biggest factor affecting cruise RPM and acceleration)
4. Choose What You Want to Calculate
A. Calculate Road Speed From RPM
If you want to know how fast you’ll be going at a certain engine RPM:
- Enter your RPM.
- Leave MPH blank.
- Hit Calculate.
The calculator will output your speed based on tire size, gear ratio, and transmission specs.
B. Calculate RPM From Road Speed
If you want to know what RPM you’ll be turning at a certain speed:
- Enter your MPH.
- Leave RPM blank.
- Hit Calculate.
The tool will find your engine speed using your other inputs.
5. Interpreting the Results
Your output will tell you:
-Estimated MPH at a given RPM, or
Estimated RPM at a given MPH
Use this to:
Fine-tune rear-end gearing
Plan highway cruising RPM
Match your tire size to your transmission
Compare different ratio setups before buying parts
6. Tips for Best Accuracy
-Enter tire diameter as precisely as possible.
-Use verified transmission ratios (aftermarket transmissions may vary).
-Remember: real-world numbers may differ slightly due to tire growth, clutch slip, and other conditions.