sat nav

[allan873]

Hi. I have a Tom Tom 300 Go in my car and there is about 5 mph. difference between the car speed and the gps the car speedo being the slower, which is right.

Allan

 

[JasonSDunks]

Don't know but my Navman iCN 520 is the same. When my speedo is showing 70, Sat Nav says I am doing 66.

And its the same at lower speeds, 40 on the speedo = 36 on satnav.

J

 

[Eddie1FromNottingham]

Hi, Allan

I have a Garmin S/P 2610 which I have to date used in four different vehicles in each the Gps unit has indicated about 5mph less than that indicated on the speedo. I have spoken to several people using different types of unit all indicating the same difference. I have been unable to establish which is correct, unit or Speedo. I would like to think that the Sat Nav is right as it's moving position is based on it's relationship with about 5 satellites. However if you watch carefully there is a slight delay in speed recording when moving off and stopping.I would be grateful if any one else could enlighten me. Give this some thought. If it's the Sat Nav thats correct vehicles Speedo's and therefore Mileometers are over recording what effect does this have over a long period on say Servicing intervals and the true milage covered by our Cars?

 

[rob_jax]

The GPS / Sat Nav system should be the most accurate.

The car speedo has to deal with manufacturing tolerances, the different rolling diameter of different manufacturers tyres, and the difference between new and worn tyres - or rather the speedo just becomes more out of step with the true speed as the tyre wears.

On my C270 the difference between new tyres and the GPS is 4km/hr - the GPS says 100km/hr and the speedo says 104km/hr. That's a difference of about 2 (and a bit) mph, and currently the difference is around 5km/hr with part worn tyres.

The Pirelli tyres currently fitted seem to be slightly larger in diameter than the nominally same sized Continentals that were fitted from new, because then there was a 6km/hr difference.

Robert

 

[Matthew_5040954]

rob_jax is correct, the reason is a legal one motor manufactures must ensure that the speedo fitted to your car doesn't cause you to break the law; therefore, as stated by rob_jax to allow for different tyres and wear your speedo will always display a speed slower then what your actually doing.

But also remember as eddie points out, there's also a delay with the sat nav, so you can't trust this, so your best to stick with the speedo, better safe the sorry.

Matthew

 

[ColinB]

I have friend who is a motorway traffic policeman. He has compared a Garmin unit against his Vascar unit and found that the Garmin was very accurate. This would bear out the previous remarks.

ColinB