Sat nav or speedo for most accurate speed?

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Sat 1 Nov 2008 00:14
Sat nav or speedo for most accurate speed? - Rattle
I was going up to Preston today in a friends clio, according to her speedo she was doing 80mph, her sat nav (tomtom) was saying 72mph. Which one of these is most likely to be accurate?

It just seems so amazing the difference. I am guessing she must have been doing 75mph.
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Sat 1 Nov 2008 07:19
Sat nav or speedo for most accurate speed? - Armitage Shanks {p}
SFAIK Sat Nav speed is way more accurate than a car speedo which is only required to be within 10% but must not underead. However, car SatNavs do not know if you you are going up or down a hill and this can lead to very slight errors, not enough to bother you with a speed camera in the area, but very small errors.
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Sat 1 Nov 2008 07:21
Sat nav or speedo for most accurate speed? - NARU
Satnav is accurate as long as the road is straight and roughly level.
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Sat 1 Nov 2008 07:32
Sat nav or speedo for most accurate speed? - Lygonos
What AS says.

Pretty much every car I've owned in the past 10yrs has overestimated speed by 5-10% on the speedo. I've checked this against motorway marker posts that are known to be very accurate (every 100m), timing a constant 62mph (100kph) against 10km of M-way (whatever keeps you awake!).

Even had both my Sat Navs running at the same time (Navman and older Tomtom) - virtually identical speeds shown and consistent with measured distances as above.

Significant gradients may have an effect as AS suggests, but Pythagoras's theorem would mean even going up/down a 1 in 10 grade (pretty sttep at 60+ mph!) the forward velocity would only be under-reading by around 1%.
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Sat 1 Nov 2008 09:03
Sat nav or speedo for most accurate speed? - Peter D
This is because the Law on speedo's specifies that they will not under-read 0% but will over-read and there is a spec but I can not find it right now. 70 mph actual, is often displayed at 74 to 77. Sat navs are basically accurate although some do not increment in 1 mph increments. If your car speedo is reading low check your tyre size and profile are compliant with the spec for the car. Regards Peter
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