What does RPM stand for? RPM stands for “Revolutions per Minute.” It’s a way of measuring the speed at which the engine revolves or spins. RPM is a measure of two things: the number of times the engine's crankshaft makes one complete rotation each minute, and simultaneously, the number of times each piston goes up and down in its cylinder. Some people call RPM “engine speed.”
An engine's RPM is indicated by a vehicle's tachometer in the instrument panel.
How the Engine Works to Create RPM
Generally, a faster spinning engine creates more power. When you press the accelerator, your car engine’s RPMs increase and its power does too, up until a point. An engine’s maximum power doesn’t always occur at its highest RPM. If you look at engine specifications, you’ll notice they state the peak horsepower figure, and then after it, the RPM at which the peak occurs. For example, engine specs might read 252 hp at 5,600 RPM. This means that the engine won't be able to make that full 252 horsepower until it's running at 5,600 RPM.
Torque typically occurs at lower RPM. Turbocharged or supercharged engines may have specs representing torque as 273 pounds-feet at 1,600 to 4,500 RPM; this means there's a wide range of engine speeds at which it can produce the maximum amount of torque.
Purpose of the Tachometer
Most car and truck dashboards include a tachometer, an indicator that informs drivers of their engine RPM. The tach is usually on the dash to the left of the speedometer (though this can vary), and it typically uses increments of thousands to measure RPM.
Typically, you see tachometer numbers from 0 to 6, 7, 8 or 9, with a “x 1,000” in the top, bottom or center of the dial face. The higher the maximum number, the more performance-oriented the engine likely is; Porsches can have maximum RPMs above 9,000 but the engine in your average family minivan can go nowhere near that number of RPMs.
If the dial is pointing to “4,” that means the engine is spinning at 4,000 revolutions per minute. The zone at the end of the tachometer range is called the redline and usually marked with an orange and then a red line. It may start at 6 or 7, meaning 6,000 or 7,000 RPM. That’s the warning sign of the limit at which you can rev the motor safely. It’s not telling you where you would find the most power.
The Dangers of Redlining
Revving past the redline, or redlining, can cause significant damage to your engine. Most modern vehicles have a rev-limiter, which prevents a driver from pushing the engine past redline.
A moment over the line is not likely to result in immediate or permanent damage, but it’s possible to cause engine wear you could avoid. Other risks include damaging your valvetrain or transmission or blowing your engine. If your tachometer dial is rising but your vehicle does not shift into its next gear properly, there might be issues with your transmission. You should seek service immediately, to avoid possible transmission failure.
Tachometers in Manual Transmission Cars
Drivers of manual transmission cars need to pay more attention to their tachometers than drivers of automatic transmissions since automatics make their own decisions on when to shift gears.
Manual transmission drivers need to know when to shift gears up or down for optimum engine efficiency and safety. Watching the tachometer helps drivers gauge when to shift gears. Shifting up before the car redlines prevents engine damage and creates a smooth transition to the higher gear. Downshifting before the RPM drop too low keeps the car from hesitating and even stalling.
Newer cars with manual transmissions are equipped with rev limiters, which help prevent engines from redlining, but the drivers are still responsible for recovering from the interruption and shifting to a higher gear. Skilled drivers can tell when to shift by listening to the engine sound and feeling whether the car engine is straining or bogging down during the drive.
Tachometers in Automatic Transmission Cars
Automatics are programmed to shift before the engine speed reaches the tachometer’s redline. The amount of pressure you exert on the gas pedal makes a difference. In normal driving, automatic transmissions shift when the engine RPM produces the ideal mix of smoothness and efficiency. For drivers of automatics, the tachometer is more of a fun thing to watch than a necessary element of the driving experience.
Benefits of Knowing Your RPM
Now that you know what RPM stands for, you should learn the RPM at which your engine generates the most power. That knowledge will help you improve your car’s performance. For manual transmission drivers, it helps with being aware of the right time to shift and benefit from the vehicle’s design.
There’s an efficiency factor at play in RPM, too. When your engine runs with lower RPM, it consumes less fuel; conversely, engines burn more fuel and air at higher RPM. That doesn’t mean your engine must live in fourth or fifth gear all the time. But if you can accelerate gradually more often, and go light on the gas pedal to avoid spiking engine speed, you should notice an improvement in your fuel economy.
Becoming familiar with your car engine’s RPM will keep you informed about how well your vehicle is functioning, make you aware of possible problems, and improve your driving habits and gas mileage. Whether you’re on the road for business or pleasure, knowing your engine RPM will enhance your driving experience.