Our car speed calculator gear ratio tool shows how each gear ratio affects vehicle speed. Understanding gear ratios is essential for performance tuning, fuel economy, and finding the right axle ratio for your driving style.
Best for highway fuel economy. Lower RPM at highway speed. Less acceleration punch but excellent cruising efficiency. Common in highway trucks and economy sedans.
Excellent balance of acceleration and highway ability. The most popular axle ratio for performance vehicles and towing trucks. Our car speed calculator default.
Strong off-the-line acceleration and towing torque. Higher RPM at highway speed uses more fuel. Popular in muscle cars and heavy-duty trucks. Reduces top speed slightly.
Maximum launch acceleration for drag racing. Very high engine RPM at highway speeds — impractical for daily driving. Our race car speed calculator mode handles this.
Our free car speed calculator online is more complete than the Spicer Parts transmission ratio calculator — because it handles all 6 gears simultaneously, shows MPH and KM/H, includes a RC car speed calculator mode for pinion/spur gear setups, a race car speed calculator mode with extended RPM range, a P-metric tire size converter, transmission efficiency adjustment, and a reverse live car speed calculator that finds RPM from target speed. It works as both a car speed calculator mph and car speed calculator km / car speed calculator km/h tool in a single interface.
The RC car speed calculator works differently from a standard vehicle calculator because RC cars use electric motors with very high RPM ratings (15,000–50,000+ RPM) and simpler gear reduction ratios. The formula: Speed (mph) = (Motor RPM × Wheel Diameter × π) ÷ (Gear Ratio × 63,360). Gear ratio = Spur teeth ÷ Pinion teeth. Example: 30,000 RPM motor, 20-tooth pinion, 80-tooth spur (4:1 ratio), 4-inch wheel: Speed = (30,000 × 4 × 3.14159) ÷ (4 × 63,360) = 56 mph. To increase RC car top speed: use a larger pinion gear (less reduction, more speed) or a smaller spur gear. To increase acceleration: smaller pinion or larger spur. Switch to RC Car mode in our calculator above.
Our car speed calculator mph, car speed calculator km, and car speed calculator km/h toggle between units with one click. The base formula always calculates in MPH using tire diameter in inches. For KM/H: km/h = mph × 1.60934. You can also use the "Both" mode to see MPH and km/h simultaneously for all gears. This is especially useful for import car tuners, international motorsport events, or anyone comparing a vehicle spec sheet that uses different units. Our online car speed calculator is the only tool that shows both units in the same bar chart.
| Speed (MPH) | Speed (KM/H) | Typical Gear | Driving Context |
|---|---|---|---|
| 0–30 mph | 0–48 km/h | 1st–2nd | City driving, parking, stop-go traffic |
| 30–60 mph | 48–97 km/h | 2nd–4th | Suburban roads, town speed limits |
| 60–80 mph | 97–129 km/h | 4th–5th | Highway cruising, US interstate speeds |
| 80–120 mph | 129–193 km/h | 5th–6th | Autobahn, race track, spirited driving |
| 120+ mph | 193+ km/h | 6th/OD | Track day, top speed run, RC car redline |
Pro tip — use the reverse calculator: The Speed → RPM reverse mode in our car speed calculator live is essential for selecting the correct axle ratio. Set your target highway speed (e.g., 65 mph) and desired cruising RPM (e.g., 1,800 RPM) in 5th or 6th gear, then adjust the axle ratio and tire size until the calculator matches. This tells you the perfect axle ratio for your specific engine's torque curve without trial and error. Our live car speed calculator also updates in real-time as you move the RPM slider.
Questions covering gear ratio speed calculation, RC car speed, tire size effects, axle ratio selection, and the speed formula.
Our car speed calculator gear ratio uses this formula: Speed (MPH) = (RPM × Tire Diameter × π) ÷ (Transmission Gear Ratio × Axle Ratio × 336). The 336 constant equals 336 = 60 min/hr × 12 in/ft × 2 (to get radius not diameter) — or more precisely, it's the unit conversion factor to get MPH from inches and RPM. For car speed calculator km/h: multiply MPH result by 1.60934. Example: 6,000 RPM, 4th gear (1.00:1), 3.73 axle, 26.8-inch tire = (6,000 × 26.8 × 3.14159) ÷ (1.00 × 3.73 × 336) = 505,019 ÷ 1,253 = 83 MPH = 134 km/h. Our car speed calculator online does all this automatically for all 6 gears.
Larger tires increase speed at the same RPM because each tire rotation covers more ground. A 1-inch increase in tire diameter increases speed by approximately 3.5–4%. Example: changing from a 27-inch tire to a 31-inch tire on the same axle ratio and RPM increases speed by 14.8%. This is why lifted trucks with larger tires need recalibrated speedometers — the speedometer still reads the old tire size. Our car speed calculator mph shows the exact speed change when you update tire diameter. Use the P-metric converter in our calculator to convert your tire size (e.g., 265/70R17 = 31.6 inches) to diameter.
Our RC car speed calculator formula: Speed (MPH) = (Motor KV × Battery Voltage × Wheel Diameter × π) ÷ (Pinion/Spur Ratio × 63,360). Or simply: (Motor RPM × Wheel Diameter × π) ÷ (Gear Ratio × 63,360) where Gear Ratio = Spur ÷ Pinion. The 63,360 = 12 inches/foot × 5,280 feet/mile. For a 3,000 KV motor on 3S LiPo (11.1V): Motor RPM = 3,000 × 11.1 = 33,300 RPM. With 20-tooth pinion, 76-tooth spur (3.8:1 ratio), 4-inch wheel: Speed = (33,300 × 4 × 3.14159) ÷ (3.8 × 63,360) = 418,407 ÷ 240,768 = 57 MPH. Select RC Car mode in our calculator and enter these values.
Use our car speed calculator online Speed → RPM reverse mode to find your ideal axle ratio: 1) Enter your target highway cruising speed (e.g., 65 mph). 2) Enter your ideal cruise RPM (e.g., 1,800–2,200 for most engines). 3) Enter your tire size. 4) Adjust axle ratio until the calculated RPM matches your target. Lower ratio (2.73:1) = lower RPM = better fuel economy. Higher ratio (4.10:1) = higher RPM = more torque multiplication = better acceleration but worse economy. For towing: 3.73–4.10. For fuel economy: 2.73–3.08. For performance balance: 3.55–3.73. For drag racing: 4.10–4.56+. Our race car speed calculator and standard modes both support this analysis.
Yes — our car speed calculator km and car speed calculator km/h toggle is at the top of the calculator. Toggle between MPH, KM/H, or Both (which shows both units in the same results bar). The conversion formula is: km/h = mph × 1.60934. All gear speeds are shown simultaneously. Our online car speed calculator is particularly useful for European and Australian users who work in km/h, or for motorsport applications that use both units (some international race tracks post speeds in km/h while pit lane communications use mph). The car speed calculator km/h mode uses the exact same gear ratio calculation — only the output unit changes.