mse wrote:

You can understand liability limitation i think - within reason

Its insurance companies which are the worst with regards to tyre and wheel sizes and not paying/claims.

On calibration I thought over reading was good...its under reading thats bad (or is that the wrong way) over reading eg saying your sped is higher than it is is expected...under reading eg saying you speed lower than it is is illegal

Correct
Thats why police usually say they allow +10% if your caught speeding