Accuracy of speed on Tomtom Display

pgnl · Occasional Visitor Joined: Jul 09, 2008 Posts: 2

Hi, I expect this issue has been discussed before, I had a quick look but couldn't find it.

I have a TomTOM Go 520 and have noticed that it in the UK it under underestimates the speed showing on the display by about 2mph. This has always puzzled me.

Anyway, I've just returned from Florida, I took the Tomtom with US maps installed on an SD card. The rental car was VW Jetta, I was interested to see that the speed shown there was 100% bang on, every time.

Is that because of some difference in geographical location, or is it simply that the speedometer on my car at home, underestimates my actual speed?

Patrick
W Mids

PaulB2005 · Posted: Tue Jun 01, 2010 10:05 am Post subject:

Hi and welcome to the Forum.

The speedometer in your car OVER-estimates your speed as it required by UK law. In other words when it shows you doing 60 mph you are probably only doing 57 mph.

In the US the situation might be different.

pgnl · Occasional Visitor Joined: Jul 09, 2008 Posts: 2

You're right it is over-estimates not under-estimate. :-)

Thanks for the reply, it's puzzled me for some time...

Patrick

robbooth · Lifetime Member Joined: Jul 18, 2006 Posts: 162

A +10% difference in speed displayed on the cars speedo is not uncommon (ie doing 70mph as shown on the gps = 77mph on speedo) - the last two cars I have had (different manufacturers) have shown this type of variation.

With my current car (VW Passat est.) I need to be going at 78mph before the gps reads 70mph.

Speedometer Info

M8TJT · Posted: Tue Jun 01, 2010 12:04 pm Post subject:

Sirf Star lll GPS spec claims 0.1M/Sec velocity accuracy. That's about 0.22MPH if my sums are correct.

TomDavison · Posted: Wed Jun 02, 2010 4:13 pm Post subject:

Speed measurement via the vehicle speedo is inherently inaccurate, since it measure (in effect) wheel rotation, and not absolute distance. Since the circumference of the wheel varies according to the diameter of the tyre (which varies slightly as the tread wears), the accuracy of the speedo will also vary over the life of the tyre.

In the UK, it is permissable for a speedo to vary between absolutely accurate, and 10% overread. It must never under-read. So you could never argue in court that you were driving at 70 on your speedo, but the speedo error meant that you were actually faster. Because of this, and to allow for variations in speedo performance, most seem to aim for an accuracy of around 5% over-read. In this way, the speedo is pretty well guaranteed to remain legal. So at 70 on the clock, most cars will be doing around 66/67mph.

Your vehicle GPS, however is based on an entirely different technology, with measures speed using a PFM circuit which is much more accurate

M8TJT · Posted: Wed Jun 02, 2010 4:34 pm Post subject:

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