A4 speedo accuracy

Anyone done any tests of how accurate the A4 speedo is (UK 2003 in this case)? I normally cruise around at ~80mph in 70 limits and have never been troubled before but I've now had some (ahem) unwanted attention and wonder
if my earlier cars read 5~10% high and my Audi doesn't.
I've read that the actual speed can be displayed by pressing a combination of buttons on the aircon control, as follows:
"A/- To access the actual speed display, start your car and then on your climate control panel press and hold-in both the Econ and Auto buttons or the Recirculation and Temperature Up buttons. This will change your outside temperature reading to Fahrenheit.
Then press the recirculation button and at the same time press the upward pointing arrow button above the recirculation button.
Your climate control screen will change to a different mode displaying 1c.
Use your temperature control + button to display code 17c. This is the actual speed display feature.
Press the recirculation button again to activate the code and to display it' s reading in mph."
However on my 2003 A4 while I can get the temp in degrees F as above, I cannot get the "1c" to appear - as I don't have an up arrow above my recirc button. Anyone got this to work?
If not it's timing those mile markers, which is a bit hit and miss.
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been
outside
it'
recirc
I don't think the procedure with the Climate Control works with the B6 A4s but I may be wrong. It definitely works on my B5 A4 as I have done it. Another way is if you have the Drivers Info System you can cruise at a given speed and then reset the average speed counter to get a reading at that time. I have done it on my A4 at a reading of 70mph on the speedo and it came out about 67mph on the computer.
Hope this helps
Richard
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No speedo is accurate. By electronic or mechanical means the revolutions per minute are calculated against the circumference of the tyres and displayed as MPH or KmPH. However, the circumference of the tyres will vary depending on the tread depth, inflation pressure and current temperature. Which is why all manufacturers design their speedos to err on the high side. Say indicating 64MPH when actual speed is just 60 MPH. This error allows for the changes in tyre circumference without ever indicating a speed lower than actual. If you car showed you doing 30MPH when your actual speed was 35MPH you could sue the manufactured if you were fined for speeding. Satellite technology i.e. sat nav is the most accurate measurement of speed which can conveniently fitted into a car. This system cannot be used as a speedo though as it only works when at least three satellites as visible, so not much use in a tunnel etc. Hope this helps.
Oldun
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I have a navigation system in my car consisting of a PDA with a GPS receiver. This shows the speed to be about 10% lower than the speedo (i.e. the speedo reads 10% high). This should be accurate.
My car is also a 2003 A4.

been
outside
it'
recirc
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Well, mine's a 2002 US so I don't know how similar they are, but I've checked mine against the radar signs they put up sometimes in the US (a sign that shows you the speed limit, and uses radar to measure and display your current speed as you go by, as gentle way of "reminding" people to slow down) I've found my speedo seems to read about 1-2 mph high at 35 mph, and 2-3 mph high at 70 mph.
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On Sat, 16 Oct 2004 06:27:25 +0000 (UTC), Douglas Siebert

My 1998 A6 Quattro agrees within 1 mile per hour to my GPS, at all speeds between 30 and 80 mph.
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writes:

My 2002 A6 Avant is about 8% out all the way according to both of my GPS units - 80mph on the speedo is about 73mph - 85mph speedo is 77mph.
I did the average speed trick when in cruise and its consistantly about 1mph fast - I asked Audi to put the speedo right or at least the same as it thinks its doling- but they said they couldn't. IMHO B****cks!
Mark
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Although the *amount* of inaccuracy varies, they all do that. Deal with it. -- C.R. Krieger (Not slip-sliding away)
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My 98 A4 was consistently 4mph slower than the speedo (according to the TomTom GPS). My TT is about 2-3mph slower than the speedo with the same GPS.
Tom
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