If it is the case that car speedometers are routinely 10% inaccurate, how does that leave a motorist who has been caught as a 'low end speeder'? If for example one were caught at 33mph in a 30mph area, then the information based on your correspondents post would suggest that the driver is in within the law (at 30mph). It would also indicate that the 'speedguns' are incorrectly calibrated if they are not detecting this error.