Most tachometer gauges (also called “tachs” or RPM gauges) are circular and have a series of numbers on them—often 0-8.
Even if you are sitting still, you will see the tachometer needle move around the gauge as you accelerate by pressing on the gas pedal. The tachometer indicates how many times the engine is rotating each minute. Multiply the number on the gauge by one thousand. For example, if the needle is sitting at 1, the engine is rotating one thousand times per minute. (If the numbers on the tachometer are multiples of 10—numbers like 20, 30, 40, and so on—then you multiply that number by one hundred instead of one thousand.) That number is how many times the engine makes one full revolution each minute—called revolutions per minute or “RPMs” for short.
It can be helpful to keep an eye on the RPM gauge. The RPM number will drop each time the transmission shifts into a higher gear to increase fuel economy. If you notice that the engine appears to be running at a higher RPM than normal, it may indicate that something is not right; that the engine is working harder than usual. Another situation when the tachometer is helpful is as you are idling. If you have a vacuum leak or a similar problem, your idling RPM will be higher than usual. Most engines should run just below the 1 mark. If the idling RPM is significantly higher, that’s a problem. You should expect to see the idling RPM a little bit higher when the engine is cold. Once the engine reaches operating temperature, you will see the gauge go back down to the 650-750 RPM range.
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