Originally Posted by Theodoric

Not necessarily. IIRC, the allowed tolerances on a speedo are -0 +10%, that is the speedo must not give too low a reading but can read up to 10% high. Assuming that a police car has a more accurate speedo and you are at at the top of the tolerance range, then you could think that you are travelling at 30mph, whereas you are actually travelling at about 27mph. The police car could therefore be travelling at an actual speed of 30mph, yet still pulling away from you. Furthermore, there is the widely reported claim (authoritative source, anyone?) that the police allow you to exceed the speed limit by 10% + 2 mph, ie 35mph for a 30 mph limit, so they could, within the rules of the game, be travelling at 35mph whereas you, keeping strictly to the speedo value might only be doing 27mph. So, the police could be travelling at 8mph faster than you, still be more or less within the law, yet be noticeably moving away from you. With higher speed limits the difference in speeds could be even greater.