Gear Ratio Calculator
Calculate the theoretical top speed in each gear at redline RPM — enter your gear ratios, final drive, and tire size.
Gear Ratio Speed Calculator
All gears at redline → km/h and mph
How It Works
Speed at redline is derived from engine RPM divided through the gear and final drive ratios to get wheel RPM, then multiplied by tyre circumference.
How to Use This Calculator
- Enter tire overall diameter in mm (use Tire Size Calculator first).
- Enter the final drive (axle/diff) ratio from the service manual.
- Enter the engine redline RPM.
- Fill in gear ratios for all forward gears.
- Click Calculate — maximum speed per gear appears.
Worked Example
Reference Table
| Gear Type | Ratio Range | Purpose |
|---|---|---|
| 1st gear | 2.5:1 – 4.0:1 | Maximum torque multiplication for starting |
| 2nd gear | 1.5:1 – 2.5:1 | Low-speed acceleration |
| 3rd gear | 1.1:1 – 1.8:1 | Mid-range performance |
| 4th gear | 0.8:1 – 1.2:1 | Direct drive range |
| 5th gear | 0.7:1 – 0.9:1 | Economy cruise |
| 6th gear | 0.6:1 – 0.8:1 | Overdrive highway |
Frequently Asked Questions
What does an overdrive gear ratio below 1:1 mean?
A ratio less than 1:1 means the output shaft spins faster than the input (less engine RPM per wheel turn). This improves fuel economy at highway speeds by letting the engine run at lower RPM.
How do I find my transmission gear ratios?
They are listed in the vehicle workshop manual under transmission specifications. Online forums for your specific model often compile them. Some tuning ECU software also reads them from the ECU.
Why is there a gap between 1st and 2nd gear that causes RPM drop?
A large ratio gap between gears means RPM drops significantly on upshift. Ideal gear spacing keeps the engine near peak torque through each upshift. Close-ratio gearboxes minimize this for racing.
Can I use this to check if my car is limited by gearing or by power?
Yes — if the calculated top speed in the highest gear is lower than the manufacturer’s claimed top speed, the car is gearing-limited. If the calculated speed exceeds the claimed speed, it is power- or aerodynamics-limited.
