The easiest way would be to follow someone with sat nav at different speeds, and work from there.

Do you mean +3 on the rear or definalty -3. My mates is 4 up on the rear and his speedo reads 10% higher than mine, and mines nearly 10% out as standard (as are many digital speedos), so his is actually about 20% out, so he's actually doing just over 80mph when his speedo shows 100mph