The four percent rule
For ordinary passenger vehicles, there’s no law in the U.S. that regulates speedometers, but U.S. manufacturers (and most Japanese as well) subscribe to a standard called SAE J1226. Your speedometer reading must be within a range of plus or minus four percent off, but that’s four percent over the entire range of the speedometer, and that range can then be shifted in what’s called a bias.
Look at the above chart based on a Scion’s technical guide. At low speeds you’re potentially going faster than the speedometer says, but at 100 mph you’re going at least three miles per hour slower.