The only time I’ve ever really seen it happen is on cars that have auto stop/start, which essentially turn themselves off anyway.
Edit: Thanks for the answers everyone! I, in fact, did not know to begin with that it is called the tachometer and measured the revolutions of the engine and not the tires!
In: 5
Because it is measuring the revolutions the engine is making, not the tires. Even at a complete stop, if the engine is on it is turning.
For all the work the engine is doing, it ends up spinning a shaft that ends up connecting to the tires and moving the car. The RPM meter measures the revolutions of that shaft, not the wheels. You’ll notice that when the car shifts gears (which change the rate at which the shaft spins) that the meter also dips rapidly before going back up. When the RPM meter goes into the red zone (spinning too fast) that’s when the transmission should shift to a new gear.
RPM stands for revolutions per minute. The tachometer (or RPM meter) measures the RPM of the motor, and not the speed of the vehicle, so anytime the motor is running the tachometer is measuring revolutions regardless of whether the vehicle is moving out not.
When the car is stopped the engine still runs at idle. Each engine has a specific idle range, lower and the car will stop, higher and you burn too much gas. The RPM gauge is measuring how fast the crankshaft spins in revolutions per minute. 700 is near or about idle, 7000 is maximum, again, dependent on the specific engine.
Imternal combustion Engines are like a domino chain. If you stop it, it will not proceed until it is started again. Since older technology needed to be able to “go” at a moments notice (eg at a stop sign) manufacturers kept the cycle going. The minimum “un-stallable” number of rpms is known as an idle, with the ability to go faster almost instantly. Newer cars may have engines that turn off, if the saving of gas and greenhouse emissions is the priority, not performance (it never was in the past). Hybrid cars don’t have this problem as they use the ICB to charge batteries and can thus take off without an engine. In short, it’s older tech and will be phased out eventually.
There’s an encoder on the motor measuring its revolutions. When the car is parked, stopped or in neutral the transmission disengages the motor from the wheels but the motor continues to turn.
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As other have said, the engine has to keep spinning when the car comes to a stop. Internal combustion engines have a minimum RPM that they need to maintain to run. Each explosion of fuel pushes the pistons and compresses the next bit of fuel to be ignited, kinda like dominos each cycle of the engine takes some energy from the one before it to keep the process going. If you slow down the engine too much you interrupt that cycle and the engine stalls and shuts off.
Your automatic transmission uses something called a torque converter to keep this from happening. It’s basically a chamber filled with fluid, and two “propellers.” One is attached to the engine and the other the wheels. The fluid lets the engine to push on the wheels, but allows them to spin at different speeds. A manual transmission requires the driver to disconnect the engine and wheels every time they push the clutch pedal. If you keep the car in gear and try and bring it to a stop with the brakes, at some point the engine will shudder and stall. The wheels are providing too much resistance for the engine to overcome.
As almost everyone else mentioned, the tachometer is measuring the engine speed–frequently the rotational speed of the crankshaft. Your speedometer is what measures your tire rotation (making an assumption of the tire’s radius). Most internal combustion engines will idle. This is mostly a legacy from when they used to use carburetors where they’d use a bit more fuel at start. Modern cars with electronic fuel injection don’t really have much of a penalty when starting the car; however, they will usually still idle.
Some newer cars will have a mode where they will shut off the engine when the car is stopped in order to avoid idling and save fuel. When you press the accelerator the car will automatically restart. In this case, the tachometer would read zero while the engine is off. I have driven one of these cars, and personally I dislike it. It is unnerving to me when the car isn’t idling, but then I still (deliberately) drive a manual transmission vehicle in the USA (for those elsewhere, almost all cars here are automatic).
Even when stopped (!Engine still running) the electric system is still on, which is driven by the engine, so it sits at a low rpm but not 0
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