Relação Marcha RPM ↔ Velocidade

Gear, RPM and speed: how they relate

The formula is v (km/h) = (RPM × π × D_tire) / (gear_ratio × final_drive) × 60/1000. Every gear comes with a fixed ratio. A typical compact car runs 1st at 3.5, 2nd at 2.0, 3rd at 1.4, 4th at 1.0 and 5th at 0.8, sitting behind a final drive near 3.5. Cruise at 60 km/h in 5th and the engine hovers around 2,000 RPM, while the redline lands somewhere between 6,000 and 7,000 RPM. Once you know where the engine burns fuel efficiently (roughly 1,800 to 3,000 RPM for gasoline), gear choice gets easier. Short gears handle hills and overtakes; tall gears keep things calm on the highway. To put numbers on it, 3,000 RPM with a total ratio of 3.5 and a tire radius of 0.31 m works out to about 100 km/h.

Applications

Driving for economy, picking a gear before a climb, lining up the gearboxes of two old manual cars, dialing in a sim racing setup, tuning a motorbike, or working out your real speed when the dash only shows RPM.

FAQ

Why does the same RPM yield different speeds in different cars? The gear ratio, the final drive and the tire diameter are all different from car to car. A truck doing 2,000 RPM in top gear crawls compared to a sports car doing 2,000 RPM in 6th.

What is the most fuel-efficient RPM? For gasoline it tends to fall around 1,800-2,500 RPM, and for diesel around 1,400-2,000 RPM. The idea is to stay in the peak-torque band using the tallest gear the road still lets you pull.

How do I find my tire diameter? It's printed on the sidewall. For a 195/65 R15, compute D = 15 × 25.4 + 2 × 195 × 0.65 mm, then divide by 1,000 to get meters.