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How accurate is TT3 speedometer
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amcluesent
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Joined: Jul 03, 2004
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Location: London

PostPosted: Wed Aug 25, 2004 11:03 pm    Post subject: How accurate is TT3 speedometer Reply with quote

I want to drive within the ACPO 'tolerance' bands for speed (i.e. nominal limit + 10% + 2mph). How accurate is the speed shown in TT3, assuming you are travelling at a constant speed on a straight, flat road)?

WHen TT3 is showing 77mph, my car speedo is showing about 82mph. They can't both be right Wink
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lbendlin
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PostPosted: Wed Aug 25, 2004 11:16 pm    Post subject: Reply with quote

the GPS based speedo is accurate to 0.1 mph . the one in your car is intentionally set to a higher speed than you actually travel.
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dlegros
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PostPosted: Thu Aug 26, 2004 1:17 am    Post subject: Reply with quote

Although IIRC, the GPS specification states that velocity on a GPS is accurate to 5mps, which is approx 11mph.

Dom
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dlegros
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PostPosted: Thu Aug 26, 2004 2:40 am    Post subject: Reply with quote

dlegros wrote:
Although IIRC, the GPS specification states that velocity on a GPS is accurate to 5mps, which is approx 11mph.

Dom


DOH! Just re-read what I typed, it is of course 0.05mps which is a more realistic 0.1mph, just as you said Lutz.

My apologies I am in the middle of a night shift at 2:30am

Dom
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RichXDA2
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PostPosted: Thu Aug 26, 2004 7:56 am    Post subject: Reply with quote

I have a Toyota RAV4 and I have been told that the speedo on these cars intentionally read 7% high! When Tom Tom reads a speed of 75mph the cars speedo reads 80mph so this would suggest that Tom Tom is accurate!
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Dave
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PostPosted: Thu Aug 26, 2004 8:38 am    Post subject: Reply with quote

All speedo's will read higher than they actually are, by law. GPS will be more accurate only if you are travelling at a constant speed on a flat surface. If you're going up a hill or down a hill speeding up slowing down, then it won't be as accurate.
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BrummyGit
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Joined: May 19, 2004
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PostPosted: Thu Aug 26, 2004 3:34 pm    Post subject: Reply with quote

The law states that your speedo has to be accurate to within 10% and MUST NOT under-read, therefore as has been most manufacturers build in a tolerance of 5-10%

Just remember as well that the ACPO guidelines say that they will prosecute at 10%+2 and so you should stick to 10%+1 to avoid prosecution. Additionally as they are only guidelines you can be prosecuted for anything above the legal limit if an area is operating zero tolerance - Mr Brunstrom's lot???
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Robin2
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PostPosted: Thu Aug 26, 2004 3:44 pm    Post subject: Reply with quote

I was about to make the same point. I am happy enough to trust the GPS speed on the level and set my cruise control to 70mph GPS speed (about 75 mph on the car speedo). But since the ACPO guidelines are not followed by all forces, I don't push it beyond that
Robin
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MikeB
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PostPosted: Thu Aug 26, 2004 5:47 pm    Post subject: Reply with quote

Dave wrote:
All speedo's will read higher than they actually are, by law.


This may only apply to the UK. I have used a number of GPS devices in rental cars both in Europe and the United States. In all cases the GPS speed was spot on with the registered Speedometer reading.
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vlak
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PostPosted: Fri Aug 27, 2004 12:55 pm    Post subject: Reply with quote

Slightly OT...

I find it interesting to learn that car speedometers are intentionally boosted up. I have just taken a driving test (to upgrade from automatic to manual) and was failed for driving above the speed limit (about 34mph) in a 30mph zone.

I was furious! I was tempted to argue that the speedometer is not accurate and that I could plug in my PocketPC with TTN3 to prove it. But then again, what's the point.

Another £40 down the drain and a further 4 months wait. What annoys me the most is that I've had a driving license and been driving for nearly 20 years (both manual and automatic, in Canada) and if my current car was automatic, I'd be entitled to drive it without having to take a test. Now the examiner has failed me twice... the first time being for NOT overtaking a bus!!!
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BrianA
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PostPosted: Fri Aug 27, 2004 8:57 pm    Post subject: Reply with quote

Just FYI, my car (Audi A4) GPS consistently shows +_5mph less than speedo ...
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sn0rg
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PostPosted: Fri Aug 27, 2004 9:16 pm    Post subject: Reply with quote

Need to bear in mind that car speedo's can be influenced by a wide variety of variables -

Tyre pressure
Wheel size
Tyre wall size
Tyre wear
etc

All of these will affect the circumference (hence the number of revolutions per mile, hence the reading on the speedometer).
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MrT
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PostPosted: Sat Aug 28, 2004 7:46 am    Post subject: Reply with quote

Dave wrote:
All speedo's will read higher than they actually are, by law. GPS will be more accurate only if you are travelling at a constant speed on a flat surface. If you're going up a hill or down a hill speeding up slowing down, then it won't be as accurate.


On a 1in 10, hill the error would be around 0.5%
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technik
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Location: Midlands UK

PostPosted: Sat Aug 28, 2004 11:42 am    Post subject: Reply with quote

I drove past one of those radar devices that shows your actual speed on a large board display, and my GPS speed was exactly the same as indicated by the radar device. ( No ticket on this occasion) :D
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Privateer
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PostPosted: Mon Aug 30, 2004 4:47 pm    Post subject: Reply with quote

Hi Vlak

vlak wrote:
I was furious! I was tempted to argue that the speedometer is not accurate and that I could plug in my PocketPC with TTN3 to prove it. But then again, what's the point.

If you had, then the examiner would probably said something along the lines of “your speedometer was reading 34mph, which was a higher indicated speed that the posted speed limit, and for the purpose of the test, the speedometer is considered accurate within the set tolerances”. Rolling Eyes

You can’t win. Sad
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