Regarding Ian Cleary's comment, it works in the states. There's been at least one case here where someone got convicted (after a fatal crash) based on some kind of computerized black box claiming they were going 80MPH or so. The defendant pointed out he had non-stock wheels, tires, and gear ratios, and the speedo (and therefore black box) were completely inaccurate. No dice -- the judge was like "the box says 80, you were doing 80" ignoring the facts of the case. Given the extremely non-stock gears and tires, he was really doing more like 40MPH -- he still was speeding and would have been convicted, but this judge threw the book at him due to not understanding how speedos work.