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How accurate is TT3 speedometer
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iancjc
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Joined: 10/02/2003 14:19:44
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PostPosted: Mon Aug 30, 2004 11:33 pm    Post subject: Reply with quote

You can't rely on the speedo ALLWAYS being 5% over.

Just swopped a Seat Alhambra for a Citroen C8:

at 70 on the dial in the seat ttn would give a reading of 65mph

same road

at 70 in the citroen TTN3 gives a reading of 69-70!mph .

Both these reading were on new tyres.

I got very used to driving at around 80mh on motorways knowing I was only doing 73 - now I stick to no more than 74 on the speedo.

One upshot of this is that over an indicated 100,000 miles the citroen will cover 100,000 rather than 92,000 and I'll save at least one service visit - funny to think a 60,000 mile warrenty could end when the car has only travelled 55,000 miles.


Makes you wonder why speedo's over read - better mph, shorter warrrenties and more servicing - who profits by this I wonder?

Ian
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Skippy
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PostPosted: Tue Aug 31, 2004 7:38 am    Post subject: Reply with quote

iancjc wrote:
One upshot of this is that over an indicated 100,000 miles the citroen will cover 100,000 rather than 92,000 and I'll save at least one service visit - funny to think a 60,000 mile warrenty could end when the car has only travelled 55,000 miles.


The odometer is different. I have a Renault Megane where the speedo reads about 5% fast whereas the odometer was spot on when I tested it using the GPS trip computer. 8O
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JJ28
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Joined: 12/08/2003 20:27:16
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PostPosted: Tue Aug 31, 2004 3:03 pm    Post subject: Reply with quote

I can relate to the Citroen. I had a C3 and that was spot on, when I drove a renault I thought the Citroen had a problem

Got a VW Bora at the moment, and that is 5% aswell.

Funny, after reading these posts I get home quicker now!
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nej
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Joined: Jun 16, 2004
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Location: London, Ingerlund

PostPosted: Tue Aug 31, 2004 5:11 pm    Post subject: Reply with quote

What I find in my car (VW Passat) is that the difference is higher the faster I am going, i.e. when I'm doing 30mph the GPS might say 29 or 28, but when the speedo says 85mph (tested on a private track, obviously Wink ) then the GPS says maybe 77mph.
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hollis_f
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Joined: 11/10/2002 06:31:24
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Location: United Kingdom

PostPosted: Thu Sep 02, 2004 1:47 pm    Post subject: Reply with quote

iancjc wrote:
Makes you wonder why speedo's over read - better mph, shorter warrrenties and more servicing - who profits by this I wonder?

Ian


It's pretty obvious why they over-read.

It's impossible to design and build any device that is 100% accurate. When the device relies on things like tyre size, pressure, time of year then it's even more impossible Wink

So, any speedo will give an inaccurate result. Suppose the car manufacturer tried to calibrate the speedo exactly (which is impossible). If they calibrated it so that the error made it under-read then they'd be in deep doo-doo. It wouldn't take long for somebody to get nicked for speeding when their speedo showed them travelling under the limit. That person would then be able to sue the car manufacturer.

By making sure all speedos over-read the manufacturers can be certain that anybody nicked for speeding will have had a speedo that read over the limit.
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bagmans911
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Joined: Mar 26, 2004
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PostPosted: Thu Sep 02, 2004 3:13 pm    Post subject: Reply with quote

That's not actually true, they could design accurate Speedos if they wished (GPS/Doppler/Light Trackers) however the cost is not in their interest. However with more and more GPS products/car instillations they may have to look more seriously at accurate speed readings, after all what does the law say about factory fitted GPS speed readouts? Should they be +10%?
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Flat_Eric
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Joined: Aug 18, 2004
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PostPosted: Thu Sep 02, 2004 3:48 pm    Post subject: Reply with quote

Not a very constructive debate really. When we all used to think 75 on the motorways was ok cos we thought our speedos were reasonably accurate, now people are going 85 instead with the lame justification that their speedos are 5-10% down.
Unless you are a really keen driver and enjoy going that extra 4 mph in a 40 or 50 zone cos your gps reads more accurately than your car speedo, if that makes you a happier person then good luck to you. Rolling Eyes
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Wilko
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Joined: Nov 17, 2003
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PostPosted: Thu Sep 02, 2004 9:48 pm    Post subject: Reply with quote

I've put the electronic dash in my previous car, and in my current car, into test mode, and theres a digital speed sensor display, and it always read 3 - 4mph down than the analogue guage.

They set the analogue guage slightly down on purpose, to allow for tolerances between each vehicle, and people fitting different tyres, under/over inflating etc etc.

If i remember correctly, speedos can read up to 10% over but are not allowed to under read the actual road speed.
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