What I've found, from a regulatory perspective, is that US regulations just require that speedometers be reasonably accurate, but EU regulations require that sp

What I've found, from a regulatory perspective, is that US regulations just require that speedometers be reasonably accurate, but EU regulations require that speedometers err in the direction of reading too high rather than too low. The EU rules must have their impact in the US because, at least anecdotally, it seems like just about every car's speedometer reads 2-3 mph high. There's also apparently a rule in the US that requires that the speedometer never read low for commercial vehicles... probably another reason that the same rule gets applied to passenger vehicles, for parts commonality. The speedometer is just based on the rotation of one of the axles, usually via a magnet and reed switch in modern cars. If you do the arithmetic you'll find that just tire wear can have a surprisingly large impact on the final speedometer indication, a few percent easily. Presumably if your tires are brand new the speedometer is much closer to accurate.