On Tuesday, 29 September 2015 16:38:32 UTC+1, charles wrote:
In article ,

So whatever a car says it's doing means little.
It can't be that difficult to fudge speedometers to under read.

The police should use their methods of reading speed which should be
checked and calibrated regualry.

To the best of my knowledge, they are. The Speedwatch kit we use is checked
against an external reference every time we use it.

That's what I'd expect I doubt what the drivers spedo says has any real relivence toi whether you've gone over the speed limit or not.
So you're speedo can say whatever it likes irrespective of tyre size.
It's the speed you are actually traveling at which is important to the police.