Final Drive Ratio Calculator
Calculate effective gear ratios and vehicle speed based on tire diameter, transmission specs, and ring/pinion gears. Visualize RPM vs Speed.
Vehicle Speed
Total Reduction
About
Optimizing a vehicle's drivetrain requires precise matching of engine power bands to road speed. Altering tire size or differential gears without compensating elsewhere often results in sluggish acceleration or excessively high highway RPM. This calculator determines the effective final drive ratio and theoretical vehicle speed. It accounts for transmission gear ratios, ring and pinion settings, and tire dimensions. Accurate drivetrain math prevents expensive mechanical mismatches and ensures speedometer calibration remains within legal tolerances. Mechanics and tuners use these figures to select the optimal setup for drag racing, towing, or highway cruising.
Formulas
The relationship between engine speed and vehicle velocity relies on the total reduction ratio. The fundamental equation for speed in miles per hour (MPH) is:
Where D represents tire diameter in inches, Rtrans is the transmission gear ratio, and Rdiff is the final drive (differential) ratio. The constant 1056 converts inches/minute to miles/hour. To find the Effective Gear Ratio (EGR) resulting from a tire size change:
Reference Data
| Transmission Model | 1st Gear | Top Gear (OD) | Common Application |
|---|---|---|---|
| Tremec T-56 Magnum | 2.66 | 0.63 | GM/Ford Performance |
| GM 4L60E | 3.06 | 0.70 | Chevy Trucks/Corvette |
| Ford 10R80 | 4.69 | 0.63 | Mustang/F-150 |
| ZF 8HP70 | 4.71 | 0.67 | BMW/Dodge/Ram |
| Toyota A340E | 2.80 | 0.70 | Supra/Tacoma |
| Honda B-Series (S80) | 3.23 | 0.85 | Integra Type R |
| Chrysler A833 | 2.66 | 1.00 | Mopar Muscle |
| BorgWarner T-10 | 2.43 | 1.00 | Classic Muscle |
| Subaru STI 6-Speed | 3.63 | 0.76 | WRX STI |
| Getrag V160 | 3.83 | 0.79 | Toyota Supra Mk4 |
| Porsche PDK (7-spd) | 3.91 | 0.62 | 911 Carrera |
| Nissan CD009 | 3.79 | 0.79 | 350Z/G35 |