RPM Calculator
Determine engine RPM at a given vehicle speed or find vehicle speed from RPM using tire diameter, transmission gear ratio, and differential ratio.
How to Use This Calculator
Choose your calculation mode
Switch between solving for engine RPM or vehicle speed using the toggle buttons.
Enter tire diameter and gear ratios
Use the overall tire diameter (including sidewall) and the gear ratios for your chosen gear and differential.
Input speed or RPM
Depending on mode, enter the known value. The calculator derives the unknown.
Review RPM vs. speed
Use the results to plan gearing changes, set shift points, or estimate cruising RPM on the highway.
Formula
RPM = (Speed × Gear Ratio × Final Drive × 336) ÷ Tire Diameter
Speed (mph) = (RPM × Tire Diameter) ÷ (Gear Ratio × Final Drive × 336)
Example: 65 mph, 0.75 overdrive, 3.73 final drive, 28" tires:
RPM = (65 × 0.75 × 3.73 × 336) ÷ 28 = 2,176 rpm
Rearranged: For 2,000 rpm, speed = (2000 × 28) ÷ (0.75 × 3.73 × 336) = 59.7 mph
About the RPM Calculator
Gearing changes and tire upgrades impact cruising RPM and performance. This calculator helps mechanics, tuners, and enthusiasts predict how modifications affect engine speed and drivability.
When to Use This Calculator
- Gearing swaps: Plan differential or transmission changes to hit desired highway RPM.
- Tire size changes: Understand how larger or smaller tires alter speedometer and RPM.
- Drag racing: Match shift points and trap speeds to keep engines in their power band.
- Fuel economy planning: Estimate cruising RPM for long commutes or road trips.
Why Use Our Calculator?
- ✅ Dual mode: Solve for RPM or vehicle speed in one interface.
- ✅ Accurate constants: Uses the standard 336 factor for US customary units.
- ✅ Quick conversions: Displays mph and km/h together.
- ✅ Mobile-ready: Perfect for shop floors, racetracks, or test drives.
Common Applications
Restorations: Calculate modern overdrive swaps in classic cars.
4x4 builds: Check crawling RPM with different axle ratios and tire sizes.
Motorsports: Fine-tune gearing and tire choices for optimal lap times.
Tips for Best Results
- Measure actual tire diameter under load for precise results—published sizes can differ.
- Remember that torque converter slippage in automatics can slightly raise real-world RPM.
- Use actual transmission gear ratios; overdrive gears are often 0.69–0.85.
- Combine with speedometer gear calculators when changing tire sizes or gear ratios.
Frequently Asked Questions
Where does the constant 336 come from?
It converts mph, gear ratios, and tire diameter in inches to RPM. The full derivation is (63360 inches/mile) ÷ (π) × 60 minutes/hour ≈ 336.
Does this work for metric units?
Inputs are in US units. To use metric values, convert tire diameter to inches and speed to mph first. Results include km/h for reference.
How accurate is the RPM estimate?
Results are very accurate for manual transmissions. Automatics with torque converters may have slight slip, raising actual RPM by a few percent.
Can I include transfer case ratios?
Yes. Multiply the transmission ratio by the transfer case ratio (e.g., 1.00 high range or 2.72 low range) and enter the combined value.