The speedometer is the round dial (or, in newer cars, the screen) in front of the driver that shows how fast the vehicle is moving right now. Every car, bus, truck and motorcycle has one, and it is the instrument the driver glances at most often, because it is the one that tells you whether you are within the speed limit. It shows speed only — how fast. It does not show how far you have travelled; that is the odometer, a different dial, usually sitting right next to it, and the two words are so close that even native speakers mix them up. Speedometer, speed. Odometer, distance. In Taiwan the dial normally reads in kilometres per hour; in the US and Britain you will see miles per hour, sometimes both rings on the same dial, which is why a car imported from America can look confusing here. One honest detail your driving instructor may not mention: car speedometers are deliberately slightly optimistic. By regulation in most countries they are allowed to read a little high but never low, so the needle saying 100 often means you are truly doing about 95. That is a safety margin, not a fault. The word is a normal everyday noun — nothing formal about it — though in casual speech people often just say "the speedo" in Britain and Australia, and Americans might say "the speed" as in "what does the speed say?"