On the RNS-E sat nav system, there's a feature called Engineering mode (press and release the NAV button, hold down the upper left button from the main knob for 5 seconds, release, then hold the upper right button for another 5 seconds). Amongst all the visible satellite info and other measurements, I noticed one for speed, shown in both mph and km/h (I think it's under the General section). Obviously the sat nav can work this out for itself, based on your position and how quickly you move between satellites.
Anyway, I thought it would be neat to find out what differences there would be between this readout and the speedometer. Speedos are renowned for over-reading - I've heard that 10% is often the norm - so you're more likely to be travelling more slowly than indicated, obviously so you don't speed. They also over-read more the faster you travel.
And the differences were quite interesting. I set cruise at an indicated 30mph along a long, straight urban road and the RNS-E reported a steady 27mph. A few days later, while thundering down the new bit of A120 past Stansted airport, I set cruise at what the needle said was 90mph, but the RNS-E said was just 83mph. Still around 10% then.
This doesn't mean we should all go out and weld our right foot to the floor, thinking an extra few mph will be safe. But I have always thought the A3 felt slower than the speedo indicated, and this kind of proves it.