Nidge2 wrote:
The meter doesn't use the tyre revolutions, it's taken from a pulse at the back of the radio.
...and the pulse is generated from......
....revolutions of the (usually rear) wheels.
Pulse generators are normally pretty accurate if they have been set up over a measured mile properly. That said, if they produce 2000 pulses on a mile exactly, that is 240 pulses more than there are yards in the mile. If on the mile a very slight deviation is made, going round a cyclist for example, it may say that there are 2002 pulses, if you have to break sharply and the back wheels lock up momentarily it may show as 1999 pulses.
The convention on meters (which I have somewhere) allows a slight variance. On my old Taxi I could not get closer than 2 yards short or 4 yards long on the mile. Both were acceptable. The odometer on that vehicle was not as accurate as the meter.