It was in the late 60s, maybe very early 70s. Sometime early in the morning, just about dawn, we were lined up at traffic lights and the police driver leant over and said "how fast are those anyway?" The upshot was that we both bombed along the A40 from Hillingdon Circus to Hoovers in Perivale. at which point I backed off. Discretion being the better part of not getting nicked, but I had about 15 mph advantage over him most of the time.