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Automotive

What is a Tach?

By Jessica Hobby
Updated May 17, 2024
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A tach is short for the word tachometer, which is a combination of the Greek and Latin words for measure and speed. A tach is used to measure angular speed of something that is rotating. The most common kind measures revolutions per minute (RPM) of a crank shaft on the engine of an automobile, aircraft, tractor or truck.

Many tachometers also have an hours meter that measures how many hours an engine has been run at a certain RPM. The hours measurement is primarily used for service intervals and is mostly seen on aircraft and tractors. For example, a tractor will need its oil changed every 30 hours or six months instead of a certain number of miles or kilometers.

A tach usually displays RPM on a calibrated analog or digital display. The display on most tachometers indicates safe operation zones, with speeds past the maximum operating speed marked in red. Operating a vehicle or aircraft past the maximum operating speed (also called “redlining”) of these safe zones may cause excessive wear and/or severe damage to the engine that includes the crankshaft, valves and pistons to possibly fail or break. The engine may explode in the most serious of situations.

All the different jobs that a tach can perform cause it to come in a variety of types, including mechanical, electric, infrared, stroboscopic, and reed-type. Tachs are used by operators and mechanics for numerous reasons. First, they alert the driver whenever there is a severe loss of engine speed, so he may make an adjustment to avoid redlining the vehicle. A tach allows a mechanic to make precise engine adjustments in the form of a tune-up and it is also used to measure the output of exhaust gases during an emissions test, which is required in many states in the United States.

German inventor Diedrich Uhlhorn is credited for inventing the tach which was first use to measure centrifugal force. Uhlhorn started using the tach to measure machines in 1817 and began to measure the engines of locomotives in 1840. In modern times, the tach is used to measure multiple things besides the RPM of an airplane, automobile, tractor or train. In medicine, tachometers are used to measure the rate of blood flow at a specific spot in the circulatory system. A tach may also be used to measure the speed of audiotape as it passes across the head in analog audio recording.

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Discussion Comments

By SkyWhisperer — On Oct 13, 2011

@NathanG - I think that's a safe assumption. If you have a boat, you can put a tach hour meter on your boat too.

These tach devices are fairly inexpensive. They show hours and rotations for your engine, just like the name implies.

My dad has one for his big boat. It helps him to know how much life he has on his engine. He does a lot of deep sea fishing. The last thing he needs is to get stuck out in the middle of the sea with a dead boat engine.

By MrMoody — On Oct 12, 2011

@NathanG - I have a light that comes on in my Honda Civic and it tells me when it’s time for the next oil change.

I am willing to bet that it uses tach technology, based on what I’ve read here. The reason I say that is that oil changes need to be done based on the amount of miles driven, and it appears the tach is capable of measuring the rotations of the engine and determining that figure.

When it hits a certain number, a digital display comes on telling me that it’s time for an oil change. That’s my theory, anyway.

By NathanG — On Oct 11, 2011

When I first read the article title I thought of the word “tachyon,” which I happen to recall from my science fiction readings.

Actually a tachyon is a subatomic particle that is supposed to move faster than light, so it has some basis in fact, but the concept has been used in science fiction to make all sorts of devices that use the principle.

As for the more mundane uses of the tach gauges, I had no idea it was used to measure blood flow. It makes sense however. Blood circulates through the body, so it’s in motion, and that’s what the tach gauge measures.

Sometimes you find these simple scientific concepts that you think belong in one context, like a gyroscope or a tach meter, and they show up in a whole host of applications in a variety of settings. I always find that interesting.

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