Slow to register Speedo

[Paula Rowe]

Hi mattsurf

I seem to recall reading that speedometers are designed to read on the high side, somewhere between 3 - 5 percent, so that at an 'indicated' 70mph, the actual road speed is in the high sixties. They are not, by Law, allowed to function the other way round, i.e. be low reading, for obvious reasons. The police also apply a rule when assessing speeding convictions which, if I recall correctly, is 10 percent plus 2mph. Thus a speed limit of 70mph translates to 79mph which, if measured by laser monitoring equipment (say), could result in a conviction. A high reading speedo' should ensure a conviction is avoided provided the indicated speed doesn't exceed local restrictions. Have you noticed that the average speed of cars on British motorways is somewhere between an indicated 70 - 80mph? Technically, many of these drivers are pushing this upper limit and are travelling above 70mph. I have often seen vehicles travelling way beyond this limit and a few get caught.

The GPS has inaccuracies too. The in-car navigational systems rely upon detecting the signals from a number of earth-orbiting (polar) satellites and then performing a series of quick triangulation calculations to determine the vehicles approximate position on the surface of the Earth. The more satellite signals that are detected the more calculations that are performed and the better the prediction of the vehicles true position. At best, this location can be determined within a few metres. Since the vehicle is moving, its position is not static (obviously) and it's the rate of change of position that determines the vehicle's velocity. If the in-car GPS takes 2 position readings, 10 seconds apart, and neither is an accurate location reading, you can appreciate that the GPS-indicated velocity will be an estimate too.

In bad weather, heavy rain (say), the water molecules in the clouds and in the rain drops block the satellite signals and only the strongest ones are detected by the in-car GPS. When the number of detected satellites fall, the positional accuracy degrades and consequently the GPS-indicated velocity degrades too. It loses its accuracy. At least 3 signals are required for GPS to work at all. Four signals is often the minimum required by most GPS systems to acquire satellite synchronisation.

I good weather and when receiving many strong signals, the in-car GPS-predicted velocity is probably more accurate that the vehicle's speedo' reading. But it doesn't stay like that because the weather in the British Isles is unpredictable.

Are Mercedes speedos more accurate than other makes? Probably not. But speedo' readings are only estimates anyway. If you stay within the indicated speed and that speed does not exceed any locally imposed restrictions, your driving license should stay free of endorsements.

If you are using your vehicle's speed control system during your testing, remember that this indicated speed will not entirely accurate either. What you are attempting to do is to compare the accuracies/innaccuracies of different speed-recording equipment. Each has its advantages and disadvantages.

If you then apply Einstein's theory of relativity then no recording/measurement we make is truly accurate, only relative.

REGARDS

Phil

I have just bought a nice w124 diesel 1995. The speedo is slow to get going and is reading about 3mph slow at 30mph. Any ideas?

 

[Blobcat]

Hello and welcome

I’ve moved your post out into a new thread for you