RPM
Enter a valid RPM > 0.
:1
Enter a valid gear ratio > 0.
The ratio for the selected gear (e.g. 3rd gear = 1.5:1)
:1
Enter a valid final drive ratio > 0.
Rear axle / differential ratio
Method 1 — Metric (e.g. 205/55 R16)
mm
%
in
Method 2 — Direct Diameter
mm
km/h
Enter a valid speed > 0.
:1
Enter a valid gear ratio.
:1
Enter a valid final drive ratio.
Metric (e.g. 205/55 R16)
mm
%
in
Vehicle Speed
—
Was this calculator helpful?
Sources & Methodology
Tyre diameter formula verified against ISO 4000-1 and SAE tyre nomenclature standards.
Tire Rack — Understanding Tyre Size
Tyre size nomenclature (width/aspect/rim format) and how tyre dimensions are derived from the sizing code.
Engineering Toolbox — Gear Ratio and Vehicle Speed
Reference for vehicle speed calculation from engine RPM, gear ratio, final drive ratio, and wheel dimensions.
Tyre diameter (mm): D = (2 × Width × Aspect/100) + Rim_inches × 25.4
Tyre circumference: C = π × D (mm) ÷ 1000 (metres)
Wheel RPM: W = Engine RPM ÷ (Gear Ratio × Final Drive)
Speed (km/h): Speed = W × C × 60 ÷ 1000
Speed (mph): Speed = W × C × 60 ÷ 1609.344
RPM from speed: RPM = (speed_ms × GR × FD × 60) ÷ C
Tyre circumference: C = π × D (mm) ÷ 1000 (metres)
Wheel RPM: W = Engine RPM ÷ (Gear Ratio × Final Drive)
Speed (km/h): Speed = W × C × 60 ÷ 1000
Speed (mph): Speed = W × C × 60 ÷ 1609.344
RPM from speed: RPM = (speed_ms × GR × FD × 60) ÷ C
⏱ Last reviewed: April 2026
How to Calculate Vehicle Speed from Gear Ratio in 2026
Vehicle speed is determined by three factors: how fast the engine turns (RPM), how much the gearbox and final drive reduce that speed (gear ratios), and how much distance each wheel revolution covers (tyre circumference). This calculator takes all three into account to give accurate speed in km/h and mph.
The Speed Formula
Speed (km/h) = (RPM × Tyre Circumference_m × 60) ÷ (Gear Ratio × Final Drive × 1000)
Tyre circumference = π × tyre diameter (m)
Tyre diameter = (2 × Width_mm × Aspect/100 + Rim_in × 25.4) ÷ 1000
Example — 205/55 R16 at 3000 RPM, gear 1.5:1, final drive 3.9:
Sidewall = 205 × 0.55 = 112.75 mm | Rim = 16 × 25.4 = 406.4 mm
Diameter = 406.4 + 2 × 112.75 = 631.9 mm = 0.6319 m
Circumference = π × 0.6319 = 1.985 m
Wheel RPM = 3000 / (1.5 × 3.9) = 512.8 RPM
Speed = 512.8 × 1.985 × 60 / 1000 = 61.1 km/h
Tyre diameter = (2 × Width_mm × Aspect/100 + Rim_in × 25.4) ÷ 1000
Example — 205/55 R16 at 3000 RPM, gear 1.5:1, final drive 3.9:
Sidewall = 205 × 0.55 = 112.75 mm | Rim = 16 × 25.4 = 406.4 mm
Diameter = 406.4 + 2 × 112.75 = 631.9 mm = 0.6319 m
Circumference = π × 0.6319 = 1.985 m
Wheel RPM = 3000 / (1.5 × 3.9) = 512.8 RPM
Speed = 512.8 × 1.985 × 60 / 1000 = 61.1 km/h
Typical Gear Ratio Speed Ranges
| Gear | Typical Ratio | Speed at 2000 RPM | Speed at 4000 RPM |
|---|---|---|---|
| 1st | 3.5 : 1 | ~18 km/h | ~35 km/h |
| 2nd | 2.2 : 1 | ~28 km/h | ~56 km/h |
| 3rd | 1.5 : 1 | ~41 km/h | ~82 km/h |
| 4th | 1.0 : 1 | ~62 km/h | ~124 km/h |
| 5th | 0.8 : 1 | ~77 km/h | ~155 km/h |
| 6th (OD) | 0.65 : 1 | ~95 km/h | ~190 km/h |
*Approximate values assuming final drive 3.9:1 and 205/55 R16 tyres. Enter your exact values above for precise results.
💡 Speedometer accuracy: Fitting larger tyres makes the speedometer read low — the car travels further per revolution than the speedo expects. Fitting smaller tyres makes the speedo read high. Use this calculator to find your actual speed versus indicated speed after a tyre size change, and to verify your speedo calibration.
Frequently Asked Questions
Speed = (RPM x tyre circumference) / (gear ratio x final drive ratio x 60), where circumference is in meters and result is in m/s. Multiply by 3.6 for km/h or 2.237 for mph. Tyre circumference = pi x tyre diameter (m). This calculator handles all steps including tyre size conversion from metric sizing format.
205 = tyre width in mm. 55 = aspect ratio (sidewall height = 55% of width = 112.75 mm). R16 = rim diameter in inches = 406.4 mm. Total diameter = 406.4 + (2 x 112.75) = 631.9 mm. Circumference = pi x 631.9 = 1985 mm = 1.985 m. This calculator reads the three numbers directly without manual conversion.
The final drive ratio is the gear reduction between the transmission output shaft and the drive wheels, located in the differential. A 4.10:1 final drive means the driveshaft turns 4.10 times per wheel revolution. Total drivetrain ratio = transmission gear ratio x final drive ratio. This combined ratio determines wheel RPM from engine RPM.
Larger tyres travel more distance per revolution, increasing actual speed for the same RPM. The speedometer, calibrated for the original tyre size, reads low — the car is faster than the display shows. Smaller tyres do the opposite. A 3% larger diameter means 3% higher actual speed versus indicated speed. Use the RPM from Speed tab to find your actual RPM at indicated speed.
Use the RPM from Speed tab, enter 100 km/h, your top gear ratio, final drive ratio, and tyre size. Typical results for a modern car in 6th gear: 1,800 to 2,400 RPM. Lower final drive ratios and taller tyres give lower cruise RPM. Higher final drive and shorter tyres give higher cruise RPM and better acceleration but worse fuel economy at speed.
Lower cruise RPM (achieved through overdrive gears and lower final drive ratios) reduces fuel consumption at highway speed. Modern 8 and 10-speed automatics have very low overdrive ratios (as low as 0.5:1) to keep cruise RPM around 1,500 to 1,800 RPM. Each 100 RPM reduction at cruise typically saves 2 to 5% fuel, depending on engine efficiency.
Enter your redline RPM and each gear ratio to find maximum speed in that gear. For example, a 7,000 RPM redline car with 205/55 R16 tyres and 3.9 final drive: 1st gear (3.5:1) = 61 km/h max, 3rd gear (1.5:1) = 143 km/h max, top gear (0.75:1) = 285 km/h max (if power limited). This shows the gear change points for maximum acceleration.
Economy cars: 3.5 to 4.5:1. Sports cars: 3.3 to 4.1:1. Trucks and SUVs: 3.7 to 4.56:1. Performance vehicles: 3.0 to 3.7:1. Lower final drive (smaller number) gives higher top speed but less acceleration. Higher final drive (larger number) gives stronger acceleration and is common in trucks needing pulling power.
Yes. Enter the primary drive ratio x gearbox ratio as the combined gear ratio, or enter the sprocket ratio in the gear ratio field. For chain-drive motorcycles, final sprocket ratio = rear sprocket teeth / front sprocket teeth. Enter this as the final drive ratio and set gear ratio to the selected gearbox ratio. Tyre diameter is entered directly in Method 2 if you know the OD.
Use the Speed from RPM tab with your new tyre size at a known cruise RPM. Compare to your speedometer reading. Alternatively, use GPS speed versus indicated speed. A 3% larger tyre (e.g., going from 205/55 R16 at 632mm diameter to 225/55 R16 at 660mm diameter) makes the speedo read 3% low. Many countries require speedometers to read within 10% of actual speed.
Most electric vehicles have a single fixed reduction gear rather than a multi-ratio gearbox. Enter gear ratio = 1:1 (or leave at 1) and enter the single reduction ratio as the final drive. For example, a Tesla Model 3 long range rear motor uses a single-speed reduction of approximately 9.0:1. Enter 1 for gear ratio and 9.0 for final drive to calculate speed at any motor RPM.
Related Calculators