On 13/02/2019 19:24, Harry Bloomfield wrote:
NY expressed precisely :
I presume manufacturers design the speedo to over-read by at least 2%
so it never under-reads even with just-legal tyres.

Yes, they make sure you can never get done for speeding, unless you
actually are speeding.

Mine seems to under read by 5 mph across the whole range from 10 - 70,
compared with a GPS speedo app.

How they read, depends on how the calculate the speed - adding a safe
percentage or simply adding 3, 4 or 5 to the value. Mine uses plus a
percentage and the mph difference increases with increased speed. Last
time I checked, it was 73mph when doing a genuine 70mph from memory.

All depends on the rolling radius of the tyre , which is not only due to
the tread depth but mainly the tyre inflation pressure. A fully inflated
tyre should give the most accurate reading.
Fortunatly the flatter the tyre the more the speedo over reads.

I think the fuzz now use a laser velometer fitted under the vehicle
which measures the true road speed relative to the vehicle.