Satnav speed accuracy

I've read here that Sat nav actual speed displayed is very accurate,
so I was surprised to see a large difference in displayed speed to
that of my speedo. I was given a Garmin Zumo 550, which I've only
stuck in the car at present. It's a nice bit of kit, well made, but I
really hope you can get other voices...

It consistently shows an over-read on the car speedo of 4mph at lower
speeds and 6mph at higher speeds. At 70 indicated this means I'm
actually only travelling at 64. I would have thought that this would
have put me at odds with other motorway users, but my speed always
seemed comparable to theirs. Similarly, when I go through those
flashing speed signs in villages etc I find that doing 42 in a 40 gets
the sign flashing (which would be 35 on the Garmin), and 39 doesn't,
which seems odd if my car speedo is that far out.

As soon as I can work out a good mount on the Blackbird I'll compare
it on that, but is the Garmin always accurate?

 

On Mon, 28 Jul 2008 10:15:06 +0100, Danny

Sounds about right.

That's because all speedos are designed to over-read, usually by
around 10%, which is what you're seeing.

Those type of 'display-your-speed' signs aren't neccessarily properly
calibrated.

I'd tend to think so.

--
_______
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Thanks, so I can drive at an indicated 76 at least without getting
nicked Bad news, though, since it makes my Isuzu Bighorn even
slower than I thought it was.

Just got to tackle the Blackbird mount, which is actually the only
reason I got the Garmin. I can navigate to Paris (and the hotel by
the Eiffel Tower) easily, but I cannot get to somewhere like Montmatre
and back in darkness without getting royally lost.

 

I'd give a try to comparation with milestones. On a highway limited to
120km/h I did that speed according to the GPS and ¡ding! "kilometer"stones
[1] appeared exactly every 30 seconds.

 

They're the ones I'm thinking off - round Meon Valley way. The one in
particular was at Bedhampton, as I happened to have the GPS on at the
time.

 

We've got quite a special one on the A1020 in a 40mph zone that
flashes your speed at you at any speed over 30mph. It's nice and
bright, and if you're the first vehicle to arrive at it in a while,
people seem to be surprised by it lighting up and flashing.

I'm still waiting for it to cause a wreck because I often see people
brake from 40 down to 30 real suddenly and the driver behind them
doesn't expect it because, hey, it's a 40 zone.

Still, they'll probably use the wreck that their poxy 'safety' sign
causes as the excuse to lower the limit from 40 to 30 down there.

 

Eat loads of space cakes and smoke loads of crack.

 

This ^

 

A recent court case for speeding went in favour of the GPS accuracy, and
the driver was let off the penalty.

 

Indeed. Although I've never been able to see the difference between my
old 3-D garmin and the later car-biased 2-D sat-navs. I used to have
them side-by-side when doing a lot of mountain driving, as it's
sometimes really useful to have the altitude displayed too.

Confidence Interval. Is that the term you're avoiding using?

--
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Stupid question on this one ... Is it just the speedo that overreads?
Or does this impact the mileage recorded as well?

 

The lowest speed I've ever set one off in a 30 was at 22mph.

There's a 'Slow Down'one that I occasionally pass, just before a blind
bend on a bridge. It flashes even when the traffic queue is doing 2
mph....

 

Or does this impact the mileage recorded as well?

My shite olde Audi has an over-reading speedo and accurated mileage
recorder. And calculates near exact fuel economy (0.2 litres per 100km
error)

 

It's cos the mechanism (at least in older vehicles) for odo and speedo
are quite different.

For the odo, the wheel turns a cable that turns a device that clicks
over the miles. It's a direct mechanical linkage.

For the speedo, the wheel turns the same cable, that turns a magnetic
disk. This spinning disk induces a <checks web for correct
terminology> eddy current in an aluminium disc attached to the speedo
needle, which, interacting with the rotating magnetic field, induces a
small torque in the disc, and hence speedo needle. This torque is
resisted by a spring.

Hopefully, you can see that getting the speedo mechanism to be
accurate is a fair bit more complicated than getting the odo mechanism
to be accurate.

Of course, everything's digital nowadays, so gawd knowa how it works.
--
Champ

ZX10R | GPz750turbo | GSX-R 600 racer
My advice as your attorney is to buy a motorcycle
To email me, neal at my domain should work.

 

And it's my guess that the number of "high" readings are being recorded. Not
a problem from a prosecution POV, but get enough "overs" and you can bet a
proper speed cam will be implanted in place of the warning tool.

It's happend on two roads I use regularly and both roads used to be 40
limits, but were dropped to 30 and had these "warning" tools. Both now have
rear facing cams.


--
Beav

VN 750
Zed 1000
OMF# 19

 

I am certainly not.

But is seems you are.
--
Champ

ZX10R | GPz750turbo | GSX-R 600 racer
My advice as your attorney is to buy a motorcycle
To email me, neal at my domain should work.

 

AIUI GPS doesn't average your speed over a distance. It uses the
doppler shift of the GPS signal
http://gpsinformation.net/main/gpsspeed.htm

 

Not quite.

"The Road Vehicles (Construction and Use) Regulations 1986, as amended,
allows the use of speedometers that meet the requirements of EC
Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC
Directive and the ECE Regulation lay down accuracy requirements to be
applied at the time of vehicle approval for speedometers. These
requirements are that the indicated speed must not be more than 10 per
cent of the true speed plus 4 km/h. In production, however, a slightly
different tolerance of 5 per cent plus 10 km/h is applied. The
requirements are also that the indicated speed must never be less than
the true speed."

http://www.theyworkforyou.com/wrans/?id=2001-03-12a.59.3

The wording's a bit ropey, but he clearly means "the indicated speed
must not differ from the true speed by more than 10% of the true speed
plus 4km/h."

The ACPO guidelines are extra-generous, as if you're exceeding a 70mph
speed limit by more than 10%+2, you're likely to have an indicated speed
closer to 90.

 

Rope wrote:

Can't think why as they work on doppler. Try driving around a
roundabout a few times and report back

 

Pixies. Tiny, digital ones.

HTH

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