A tachometer shows engine speed in revolutions per minute. It sits next to the speedometer on most sporty cars. On a manual transmission car, it tells the driver when to shift. On an automatic, it is mostly informational, though it helps with towing and diagnosing problems.

Mechanical tachometers used a cable driven by the ignition distributor or a spinning magnet. Electronic ones count ignition pulses or read a crankshaft position sensor. The display can be an analog needle, a digital number, or a bar graph. Some modern cars omit the tachometer entirely because the engine is so quiet and the transmission so smart.

What the numbers mean

• Idle: usually 600 to 900 RPM.
• Peak torque: often 1,500 to 4,000 RPM.
• Peak horsepower: often 5,000 to 7,000 RPM.
• Redline: maximum safe speed, usually 6,000 to 8,000 RPM on petrol engines.

Bouncing off the rev limiter cuts fuel or spark to protect the engine. Downshifting too aggressively can over-rev and float the valves, which is expensive. A tachometer is a simple instrument, but it gives a direct window into what the engine is doing at any moment.