With regards SatNav, my Tomtom, on my Nokia and my 520T, my speedo has always read about 5 miles over, and also on the flashing road signs that tell you your speed, but the SatNav and the road signs read the same.
Om my latest Lag, it reads the same as the SatNav and the roadsigns. this was quite a surprise.
Haven't we gone off topic somewhat here though? lol