🚗 Transmission Calculator
Calculate Vehicle Speed from Gear Ratios
Example: 1st gear 3.5:1, 2nd 2.0:1, 3rd 1.5:1, 4th 1.0:1, 5th 0.8:1
Typical: 24-30 inches for passenger vehicles
How to Use This Calculator
Enter Engine RPM
Input the engine RPM at which you want to calculate vehicle speed.
Enter Transmission Ratio
Enter the transmission gear ratio for the gear you're calculating (e.g., 3.5 for 1st gear, 1.0 for 4th gear, 0.8 for overdrive).
Enter Final Drive Ratio
Input the final drive ratio (axle ratio, differential ratio). This is typically stamped on the differential or in vehicle specifications.
Enter Tire Diameter
Enter the tire diameter in inches. You can calculate this from tire size (e.g., P225/60R16) or measure directly.
Calculate
Click calculate to get overall gear ratio, wheel RPM, and vehicle speed in mph and km/h.
Formula
Overall Ratio = Transmission Ratio × Final Drive Ratio
Wheel RPM = Engine RPM / Overall Ratio
Speed (mph) = (Wheel RPM × Tire Circumference × 60) / (5280 × 12)
Where:
- Overall Ratio = Combined gear reduction
- Transmission Ratio = Gear ratio in transmission
- Final Drive Ratio = Axle/differential ratio
- Tire Circumference = π × Tire diameter (inches)
- 60 = Minutes to hours conversion
- 5280 = Feet to miles conversion
- 12 = Inches to feet conversion
Example Calculation
If you have:
- Engine RPM: 3000
- Transmission ratio: 1.0 (4th gear)
- Final drive: 3.73
- Tire diameter: 27 inches
Calculation:
Overall Ratio = 1.0 × 3.73 = 3.73:1
Wheel RPM = 3000 / 3.73 = 804 RPM
Tire Circumference = π × 27 = 84.82 inches
Speed = (804 × 84.82 × 60) / (5280 × 12) = 64.7 mph
About Transmission Calculator
The Transmission Calculator is an essential tool for automotive enthusiasts, mechanics, and engineers working with vehicle drivetrains. It calculates vehicle speed from engine RPM, transmission gear ratio, final drive ratio, and tire diameter. This calculator helps understand how gear ratios affect vehicle performance and is useful for gear selection, performance tuning, and drivetrain modifications.
When to Use This Calculator
- Performance Tuning: Calculate speeds at different RPMs and gears
- Gear Selection: Compare different final drive ratios for desired performance
- Tire Selection: Understand how tire size changes affect speed and gearing
- Drivetrain Modifications: Plan transmission or axle ratio changes
- Racing Applications: Optimize gearing for track conditions
Why Use Our Calculator?
- ✅ Instant Results: Get speed calculations immediately
- ✅ Easy to Use: Simple interface requiring only RPM, ratios, and tire size
- ✅ Multiple Units: Results displayed in both mph and km/h
- ✅ 100% Free: No registration or payment required
- ✅ Accurate: Uses standard automotive engineering formulas
- ✅ Educational: Shows overall ratio and wheel RPM for understanding
Common Applications
Performance Vehicles: Calculate top speed in each gear and determine optimal shift points for maximum performance.
Off-Road Vehicles: Determine gear ratios for different terrain conditions and calculate crawl ratios for low-speed applications.
Towing Applications: Select appropriate final drive ratios for vehicles that tow heavy loads, balancing power and fuel economy.
Fuel Economy Tuning: Optimize gear ratios and tire sizes to achieve better fuel economy while maintaining acceptable performance.
Tips for Accurate Results
- Use actual tire diameter - this can be calculated from tire size or measured with weight on tire
- Account for tire growth at speed - high-speed tires expand slightly, increasing effective diameter
- Remember that lower numerical ratios (like 0.8:1) are overdrive gears (faster wheel speed)
- Higher numerical ratios (like 3.5:1) provide more torque multiplication but lower speed
- Actual speeds may vary slightly due to tire slip, transmission slip, and other factors
Frequently Asked Questions
How do I find my transmission gear ratios?
Transmission gear ratios are typically found in vehicle service manuals, owner's manuals, or manufacturer specifications. Common 5-speed manual ratios: 1st 3.5-4.0:1, 2nd 2.0-2.5:1, 3rd 1.3-1.5:1, 4th 1.0:1, 5th 0.7-0.9:1.
What is final drive ratio?
Final drive ratio (also called axle ratio or differential ratio) is the gear reduction in the differential/axle assembly. Common ratios range from 2.73:1 (highway gears) to 4.10:1 or higher (performance/towing gears). Lower numbers provide higher speeds but less torque.
How does tire size affect speed?
Larger tires increase vehicle speed at the same RPM (they travel farther per revolution). Smaller tires decrease speed. Changing tire size effectively changes final drive ratio - larger tires act like a numerically lower gear ratio.
What is a good gear ratio for my vehicle?
It depends on use. Lower ratios (2.73-3.23) favor fuel economy and highway speed. Higher ratios (3.73-4.10+) favor acceleration and towing. Most passenger vehicles use 3.08-3.73 ratios as a compromise.
Why doesn't my actual speed match the calculation?
Actual speeds may differ due to: tire wear reducing diameter, tire slip, transmission/converter slip in automatics, tire growth at high speed, speedometer calibration errors, or incorrect input values. The calculation provides theoretical speed.