[entwisi]

You will find that 75mph indicated on a car speedo is an actual 70 or sometimes less. My GPS shows my cars speedo at 76 for an actual 70, 84 = actual 77 (speed limit +10% which is the actual speed coppers will start to look at issueing a ticket)

Some cars are even further out.

So sitting at an indicated 75 busman will probably not have been speeding.