You've written a lot about speedometers over-reading a car's true speed. I recently hired a car, an i30, and was driving through a village with a speed indicator sign and the sign indicated the exact speed that the speedo was reading (I would say there would have been no more than a few milliseconds between me seeing the sign and reading the speedo).

I guess these signs use radar to judge a car's speed and can be considered accurate. Have you heard that a Hyundai's speedo accurately records the true speed or is it not so clear-cut as that?