While many gauges have disappeared from the dashboard of modern cars, the tachometer—or rev counter—has survived for a reason. It's fun to watch and gives you a good idea of the engine's power delivery. If the pointer goes over or speeds up toward the red line of a tachometer, it's time to get the car serviced before damage occurs.
The rpm of your engine measures how fast the crankshaft rotates each minute and how many times the pistons go up and down in the cylinder. It also tells you how much power the engine can generate up to a certain level—which is why most car manufacturers specify this peak horsepower and rpm on their performance specifications sheets.
When you press on the gas pedal, your car's rpm increases to match its maximum power output, then drops as it shifts gears to keep driving at that speed. This is why most drivers who use automatic transmissions prefer to keep the car at a lower rpm when accelerating than they would if they were using manual transmission.
If you notice that your vehicle's rpm starts to increase while driving and the engine isn't producing more power, it may be a sign of spark plug issues. This is why it's important to read your owner's manual and check the recommended maintenance schedule in your car.