I got given a really crappy Toyota Echo when I took my car in for its last service, that speedo was bang on the money (with Waze) so I just use my phone to tell me how fast I'm really going now... Very useful for cops and speed traps anyway, couldn't drive without it now...
When we first moved out here we had a corolla rental that over read by 5km/h.
It was pretty annoying driving into central and getting people hassling you to overtake when you were sat doing bang on the speed limit on cruise