sat nav

[chris1970]

I have just recently travelled back from Germany using a satnav system which i was very impressed with ,but i noticed that the satnav displayed my current speed this was contantly about 5-8 mph lower than the actual speedometer on my car .How acurate is the sat nav and how long will it be before people start disputing speeding fine with tyhese as evidence

[Oneeyedvic]

I believe that most cars speedometers are actually slow by around 10% or 5-8 mph when travelling at 70mph.. The gps system is more accurate.

However, if you have been speeding this means that if you are actually caught at 80mph this probably means that you thought you were doing 88mph - so doubt that you will complain or try to dispute!

[idalovett]

I am not sure about the accuracy. Does it depend on the type of satnav you get? My TomTom was showing the same speed as my dashboard display and I saw no sense in being able to see it twice so disabled the satnav from showing it.

[gumboot]

I've had reason to look into this recently and I'm afraid that Oneyedvic and idalovett are mistaken.

Sat-nav systems measure your speed over the surface of the earth without taking contours etc into account. In effect, they work by using line of sight distance and time of travel between points to work out the average speed of your vehicle at a given moment.

None of us travel from A to B in this manner, because no road is perfectly flat, straight and contourless. The only accurate way to measure speed over a given distance is via a speedometer. Because of this, what Oneyedvic said in the second paragraph needs swapping around ie if your sat-nav says you're doing 80mph, you're probably doing 88mph. Your experiences with this 5-8 mph drop chris1970 sound about right.

The police will not accept a sat-nav reading as an accurate reading because of what I said earlier. They rely on the speedometer reading. Furthermore chris1970, there's been at least two test cases in the courts about this where a driver tried to claim his sat-nav showed he was travelling below the speed limit.

Expert witnesses were called on behalf of the prosecution and they demolished the case for the defence on the basis of this curvature of the earth. It's now gone down as case-law, which means anyone trying their luck with this defence in the future is in for a nasty surprise.

I'm not disputing that in the right circumstances, they can show the same speed as your speedometer. The problem is that they don't do it all the time and this innacuracy and uncertainty is enough to get booked. Besides, you haven't got a chance if your caught in a radar trap.

[chris1970]

cheers gumboot just the sort of detailed answer i was looking for great detail thanks.

[Oneeyedvic]

Sorry, give me one star if you like but here you go:

West Lothian police confirm that manufacturers will make speedometers show an over speed - eg 32mph when you are actually tavelling 30mph. http://www.westlothiannews.co.uk/Articles/traf ficpolice.html

All these GPS users agree with my answer: http://uk.answers.yahoo.com/question/index?qid =20070131084744AA91HSH
http://www.theanswerbank.co.uk/Travel/Question 179262.html
http://www.expansys.com/ft.aspx?i=112333&threa d=232

You even showed it in yor own answer - you say "the satnav dispalyed my current speed this was contantly about 5-8 mph lower than the actual speedometer on my car ." - so if your speedometer was showing 80 mph, the sat nav would have been showing 72mph.

Gumboot states: "ie if your sat-nav says you're doing 80mph, you're probably doing 88mph. " - not according to you it isn't.

The only thing I would agree on is that you couldn't use this in a court of law - but as I stated in my first answer, if you were clocked at 90 mph by the police, your sat nav would in all likeliood state 90mph but your speedometer would state 95mph+