My SatNav indicates that my speedo (at motorway speeds at least) is typically a couple of mph too low. So sticking thicker tyres on for a more comfortable ride might actually bring them into agreement?
Speedo's are DESIGNED to show a higher speed than actual, rather than under read .. so when it says 70 actual speed is likely to be 67 ish .. this is because the law does not allow them to underread ... at all.
Satnavs CAN be accurate under certain conditions, and a lot depends on the sampling rate of the device. If you are traveling in a dead straight line, perfectly level, at a constant speed .. then chances are the satnav readout is accurate. if ANY of those 3 details are varying (going around a corner, climbing a hill, speed changing) then the satnav will become inaccurate to some degree.
This is quite simply due to how it works out your speed .. it does NOT "measure" speed in any way at all ... it calculates a position (A) then at a specific time (T) later it calculates another (B). It then calculates the distance between A & B - in a straight line - and it knows the time taken .. so calculates a speed. If the satnav is a cheap one, or one designed for walking not driving, then the time T is a "large" one .. and if you were, for example, driving a curve, the satnav will assume you cut the corner, so it will "think" you have travelled a shorter distance .. thus your speed will indicate LOWER than actual ........ so if you thought you were doing 70 you might be actually doing 75 ... bye bye licence ....
