MFactory Gear & Speed Calculator
Enter your setup values to estimate vehicle speed in each gear at a target RPM. This is useful for planning a street, drag, or track gearbox.
Tip: 205/50R15 is about 23.1", 225/45R17 is about 25.0".
Transmission Gear Ratios
Note: This calculator provides theoretical values and does not account for clutch slip, tire growth, drivetrain loss, or aerodynamic drag.
How this mfactory gear calculator helps
A good gear setup can transform how a car feels. Whether you're installing an MFactory final drive or planning a full gear stack, the most important question is simple: what speed will each gear reach at my chosen RPM? This calculator gives you that answer immediately.
Instead of guessing, you can compare ratio combinations, see where shifts happen, and find out whether your transmission is too short, too long, or just right for your use case.
The formula behind the calculator
The speed estimate in miles per hour uses a standard drivetrain relationship:
- MPH = (RPM × Tire Diameter) / (Gear Ratio × Final Drive × 336)
- KM/H = MPH × 1.60934
The constant 336 converts tire diameter and wheel speed into road speed for imperial units. If you increase final drive or gear ratio values, top speed per gear decreases, but acceleration potential increases.
Choosing ratios for your goal
Street performance
For a daily-driven build, you usually want smooth spacing and reasonable highway RPM. A very short final drive can feel exciting in city driving but may increase noise and fuel consumption on long trips.
Drag racing
Drag setups often prioritize aggressive multiplication in lower gears while making sure you don't run out of gear before the finish line. Use the calculator to verify trap speed in your chosen gear at redline.
Road course / autocross
Track and autocross drivers need ratios that keep the engine in the powerband after every shift. A tight gear spread can reduce RPM drop and improve corner-exit performance.
Common tuning mistakes to avoid
- Using tire diameter guesses that are far from real mounted size.
- Focusing only on 1st gear acceleration and ignoring shift recovery.
- Selecting a final drive that causes an extra shift in critical sections of a race.
- Not checking highway cruise RPM for street-driven cars.
Practical workflow
- Enter real tire diameter and your true shift RPM.
- Input the target final drive and all transmission gears.
- Review per-gear speed and RPM drop after each shift.
- Adjust one variable at a time until the spread fits your usage.
Final thoughts
A ratio change is one of the highest-impact drivetrain modifications you can make. With this mfactory gear calculator, you can plan your setup with clarity before buying parts. If you're comparing multiple final drives or gear sets, save your outputs and line them up side by side. The best setup is the one that matches your engine, tire, and intended driving environment.