A tachometer is the instrument that shows how fast an engine is turning. Not how fast the car is going down the road — that is the speedometer — but how many times the engine spins round in one minute. The number is given in RPM, revolutions per minute, and on a car it is usually a round dial next to the speedometer with the numbers 1 to 8, each standing for a thousand. Drivers care about it mainly when the car has a manual gearbox: if the needle climbs into the red part at the top, the engine is turning too fast and you should change up a gear. In an automatic car many people never look at it at all. Motorbikes, boats and aircraft have them too, and so do factory machines — anywhere someone needs to know the turning speed of something. In speech almost everyone shortens it to 'tach' (American, said 'tack') or 'rev counter' (British). 'Tachometer' itself is the full, slightly technical form you see in a manual or a car review. Be careful with the stress: it is ta-KOM-e-ter, with the strong beat on the second part, the same shape as 'thermometer' and 'speedometer'.

轉速表、引擎轉速錶（就是儀表板上寫 RPM 的那個圓錶）。台灣開車的人常直接說「轉速表」或「看轉速」；它跟「時速表」speedometer 是兩個不同的錶。