Which is correct?

[malc]

I have recently fitted a new Kenwood Sat nav ... by VW dealer.

When I am driving the sat nav speed is showing 4 mph less than my speedo. This is across all speeds. I feel the sat nav is right because it tallies with the road signs showing your speed.

Which one is correct and how can I get them to synch?

Could this affect my mpg readings and mileage recordings?

 

[Andresz]

Hello
GPS is the one to be accurate.
On board displays are usually calibrated for speed higher than real. Why?, probably to avoid situations where you drive with higher than intended speed. Lower speed means more security and no liability of the manufacturer.
The speedo will always have an inaccuracy, at least for the reason of changing thickness of the tire tread. Obviously it would affect the mpg and mileage calculations.
Cheers
Andrzej

 

[malc]

Thanks Andrez

 

[Goldie]

I have recently fitted a new Kenwood Sat nav ... by VW dealer.

When I am driving the sat nav speed is showing 4 mph less than my speedo. This is across all speeds. I feel the sat nav is right because it tallies with the road signs showing your speed.

Which one is correct and how can I get them to synch?

Could this affect my mpg readings and mileage recordings?

This may explain things: http://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/

Andy

 

[gatvol]

In my experience the speedos in cars seem to be set higher than the tue speed. A friend's wife did a speed management course run by the police as an alternative to getting three points for speeding and this was mentioned at the time.

GPS is probably more accurate but can give problems when travelling up and down hills. If you have cruise you could set it, have your gps on and drive past those police "what speed you are doing with a smiley face type signs" you see in some areas. Best to find one on a flat road. My guess is that your gps will be closer to what the smiley face camera tells you that you are doing.

 

[Andy]

GPS is more accurate, but you need to be doing a constant speed to see what the difference is.