Manufacturers are required to 'build in' a margin of plus 5%, speeding tickets not issued at less than 5% + 2mph. **
This means that at 70mph you will be doing an actual 64.5mph.
That of course only works when the vehicle is the original setup and tyres are fully treaded, as my 95 runs on oversized tyres my speedo now reads at minus 5%, so at 70 I am travelling at 73.5mph.
So your speedo sounds about right.
** Except 30mph zones, where there is no exemption, 31mph = ticket/fine etc. This is because your speedo would be showing at least 33.5mph.
Pete