Transmission Gear Ratio
Calculate vehicle speed from engine RPM, transmission gear ratio, differential ratio, and tire diameter. Free automotive calculator with charts and breakdowns.
About This Calculator
The Transmission Gear Ratio Calculator helps automotive enthusiasts, mechanics, and engineers calculate vehicle speed from engine RPM and drivetrain specifications. Whether you are tuning a race car, planning a gear swap, or simply curious about your vehicle's performance, this calculator tells you exactly how fast you are going at any RPM in any gear.
How It Works
The calculator uses the standard transmission formula: Speed (km/h) = Engine RPM × Tire Diameter (mm) × π × 60 / (Transmission Gear Ratio × Differential Gear Ratio × 1,000,000). The overall gear ratio is the product of the transmission gear ratio and the differential (final drive) ratio. Wheel RPM is calculated by dividing engine RPM by the overall ratio. The results include speed in both km/h and mph, wheel RPM, and a Speed vs RPM chart showing how speed varies across the RPM range for your gearing setup.
Why Gear Ratios Matter
Gear ratios determine the balance between acceleration and top speed. A lower (numerically smaller) overall ratio gives higher top speed at lower RPM — ideal for highway cruising and fuel economy. A higher (numerically larger) overall ratio multiplies torque for quicker acceleration — ideal for towing, off-roading, or track driving. This calculator helps you find the sweet spot for your specific use case, whether it is daily driving, motorsport, or commercial vehicle operation.
Regional Notes
India: Indian cars typically use smaller engines (1.0L–2.0L) with 5-speed or 6-speed manuals. Common differential ratios range from 3.5:1 to 4.2:1. Tire diameters average 24–26 inches. Typical highway cruising RPM is 2000–3000 rpm at 100 km/h.
United States: US vehicles include larger engines and trucks with differential ratios ranging from 2.7:1 (fuel economy) to 4.5:1 (towing). Many modern cars use 6–10 speed automatics with overdrive gears as low as 0.5:1. Tire diameters range from 24–35 inches depending on vehicle type.
United Kingdom: UK cars are similar to India in engine size trends. Manual transmissions dominate with 5–6 speeds. Differential ratios typically range 3.2:1–4.0:1. Speed limits are in mph (30, 60, 70 mph), so the mph output is especially useful for UK drivers.
Frequently Asked Questions
How do I calculate vehicle speed from engine RPM and gear ratios?
Vehicle speed is calculated using the formula: Speed (km/h) = Engine RPM × Tire Diameter (mm) × π × 60 / (Transmission Gear Ratio × Differential Gear Ratio × 1,000,000). Enter your engine RPM, transmission gear ratio, differential (final drive) ratio, and tire diameter in inches, then click Calculate to get speed in km/h and mph.
What is the difference between transmission gear ratio and differential gear ratio?
The transmission gear ratio is the ratio inside your gearbox that determines how engine RPM is converted to driveshaft RPM for each gear. The differential (final drive) ratio is the ratio in your axle differential that further reduces RPM before it reaches the wheels. Together they form the overall gear ratio, which determines vehicle speed at any given engine RPM. Most passenger cars have differential ratios between 2.5:1 and 4.5:1.
How do I find my car's transmission gear ratio and differential ratio?
You can find your vehicle's gear ratios in the owner's manual, service manual, or online specifications database. Common sources include your car manufacturer's website, enthusiast forums, or databases like carspecs.info. The transmission gear ratio for each gear is listed, and the final drive (differential) ratio is a single number for your vehicle model.
What tire diameter should I use for my car?
Tire diameter can be calculated from your tire size markings. For example, a 225/45R17 tire has a sidewall height of 225 × 0.45 = 101.25 mm. Total diameter = (2 × sidewall height) / 25.4 + wheel diameter = (2 × 101.25) / 25.4 + 17 ≈ 25 inches. Most passenger cars use tire diameters between 24 and 28 inches.
What happens if I change my tire diameter?
Increasing tire diameter raises your vehicle's top speed at the same RPM but reduces acceleration (torque at the wheels). Decreasing tire diameter improves acceleration but lowers top speed. Your speedometer reading also changes — larger tires make the speedometer read slower than actual speed, while smaller tires make it read faster. The formula used by this calculator accounts for tire diameter changes.
How does the overall gear ratio affect fuel economy?
A lower overall gear ratio (taller gearing) means lower engine RPM at highway speeds, which typically improves fuel economy because the engine operates more efficiently. A higher overall ratio (shorter gearing) keeps RPM higher, providing better acceleration but usually worse fuel economy. Modern cars use taller gears in top gear (overdrive) with ratios below 1.0:1 to optimize highway fuel efficiency.
Is this transmission calculator free to use?
Yes, this transmission gear ratio calculator is completely free to use with no registration required. You can save your inputs as a shareable URL to revisit or share your calculations with others.