What is the relationship between gear, RPM, and speed?
3 Answers
The relationship between gear, RPM, and speed is as follows: 1. Mutual matching: The engine RPM, gear, and vehicle speed should match. All car engines have an RPM range where they output maximum torque. When the transmission is in a certain gear (except the highest gear), and the engine reaches the RPM for maximum torque output (e.g., 3000 RPM), the vehicle speed reaches the maximum speed that gear can handle. At this point, you should shift up. 2. Depends on road conditions: Taking the Lova as an example, when driving at 60 km/h in third gear with the engine RPM approaching 3000, you should shift to fourth gear. Of course, you can also shift around 2600 RPM, depending on whether the current road conditions allow you to continue accelerating smoothly. If you can't continue accelerating, release the throttle to decelerate and follow traffic.
I've been driving a taxi for fifteen years, and the coordination between gear, RPM, and speed is like the hands and feet of driving. How fast the engine is spinning is shown by the tachometer, and how fast the car is moving is shown by the speedometer. Shifting gears is about making them dance in harmony. For example, in second gear, you might hit 3000 RPM at just 40 km/h, but at the same RPM in fifth gear, you can reach 100 km/h. Why shift at 2000 RPM in a manual transmission? It saves fuel and protects the engine. When downshifting to overtake, the pushback feeling at 5000 RPM in third gear is much stronger than in fourth. If the engine screams without speeding up, it's time to upshift—experienced drivers can tell which gear to shift to just by the sound. Never shift into second gear at 60 km/h; the engine will roar and burn oil. Remember to use lower gears on uphill climbs, keeping the RPM around 3500 for more power. Beginners should watch the tachometer closely and shift before the needle hits the red zone.
From a mechanical principle perspective, a transmission's gears function similarly to those on a geared bicycle. The engine's crankshaft outputs rotational speed, which is then modified through different combinations of gear sizes in the transmission before being transmitted to the wheels to propel the vehicle. Lower gears like 1st and 2nd use a high gear ratio—the engine turns 3 times for every 1 wheel rotation, resulting in slower speed but higher torque, making it powerful for climbing hills. Higher gears like 5th and 6th employ a low gear ratio—the engine turns once for every 1.5 wheel rotations, allowing faster speed at the same engine RPM. This gear ratio determines the conversion relationship between engine speed and vehicle speed. Automatic transmissions follow this same principle in their shift logic—as speed increases, the computer upshifts to lower engine RPM for better fuel efficiency. A common misconception is equating high RPM with high speed, when in fact at 5000 RPM, 4th gear may deliver twice the speed of 2nd gear. For optimal fuel economy, maintaining 1500-2500 RPM during daily driving is recommended.
