Try telling the magistrate that and see how far you get.
Depending on how much over you were, coupled with the road quality etc, potentially you could get away with it.
Anyway, it only really matters if you get caught. In which case, the police's measurements allow a margin for error anyway.
They used to in the UK, however recently they implemented a zero tolerance policy, which thanks to the accuracy of speed cameras meant that in some areas (notably Avon and Somerset) people were being hit for points and fines for going .5 of a mile an hour over the limit.
Due to the outcry and the fact that it was physically impossible for the average motorist to get readings accurate enough not to get slapped, leeway was re-introduced very quietly.