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digital Regular Visitor
Joined: Nov 29, 2004 Posts: 88 Location: Leeds, UK
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Posted: Fri Jun 04, 2010 11:54 pm Post subject: TT 930 speed accuracy. |
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I've just taken delivery of a new Vauxhall Astra and the speedo seems to be hopelessly out so I've checked it with my TT 930.
There's quite a discrepancy:
Car says 70, TT says62
Car says 50, TT says 45
etc.
Before I contact Vauxhall it would be useful to know just how accurate the TT930 is. Anyone know? And, yes, I have searched but the only results I have got are along the lines of 'should be, could be, may be...'.
TIA _________________ Frank |
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Anita Pocket GPS Moderator
Joined: Mar 15, 2006 Posts: 3219 Location: Windlesham, Surrey
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Posted: Sat Jun 05, 2010 12:21 am Post subject: |
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The GPS speed is correct. By law in the UK cars' speedos cannot under-read, so they are set to over-read to allow for tyre wear etc. Mine is out by around 10%.
This has been discussed here many times, but it's difficult to hit on the right search terms. Here is the most recent thread. _________________ Anita
TomTom VIA 135 - App 12.075
UK map 1130.12368
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digital Regular Visitor
Joined: Nov 29, 2004 Posts: 88 Location: Leeds, UK
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Posted: Sat Jun 05, 2010 12:36 am Post subject: |
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Thanks.
While you say the GPS speed is 'correct', it cannot be absolutely correct, there must be a margin of error. It is that that I am searching for.
In addition, as the tyres wear I would expect the car's speedo under-reading error to increase and so, as mine is already close to or over the legal limit of 10%, it will become illegal. _________________ Frank |
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Anita Pocket GPS Moderator
Joined: Mar 15, 2006 Posts: 3219 Location: Windlesham, Surrey
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Posted: Sat Jun 05, 2010 12:47 am Post subject: |
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digital wrote: | While you say the GPS speed is 'correct', it cannot be absolutely correct, there must be a margin of error. It is that that I am searching for. |
OK, the GPS speed is considerably more accurate than your car's speedo.
It's beyond my personal knowledge, but in the thread I linked to M8TJT said "Sirf Star lll GPS spec claims 0.1M/Sec velocity accuracy. That's about 0.22MPH if my sums are correct." I believe his sums are usually correct.
Edited to add: I've just done the sum and get the same answer. _________________ Anita
TomTom VIA 135 - App 12.075
UK map 1130.12368
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mikealder Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Sat Jun 05, 2010 12:57 am Post subject: |
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From the figures you have supplied even taking into account tyre wear, if your GPS speed ever matches your speedo you will be running on slicks i.e. totally bald tyres.
If you don't check tyre tread depth weekly please take a close look at the rubber on your vehicle if the speedo ever gets within a couple of MPH of the GPS speed, modern cars take there speed from pulses gennerated at a fixed rate per revolution of the wheel so tyre size, pressure, heat and wear can all contribute to affect the displayed speed on the cars speedo, GPS derived speed is position dependent with a one second update rate and is devoid of tyre fluctuations - Mike |
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digital Regular Visitor
Joined: Nov 29, 2004 Posts: 88 Location: Leeds, UK
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Posted: Sat Jun 05, 2010 1:22 am Post subject: |
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As the tyre wears, the diameter of the wheel reduces and, as a consequence, the circumference reduces. So, for every revolution of the wheel the car will travel a shorter distance. Meanwhile the rotation sensor which feeds the speedo 'thinks' the car has travelled the distance appropriate to the original wheel diameter. It continues to display x mph while the car has actually travelled at a speed of x mph minus an amount appropriate to the reduction in circumference. Today, on new tyres, the car was already travelling slower than the speedo showed and so when the tyres wear it will be travelling even slower than that. The difference between true speed (as indicated by TT) and the speedo will increase. The error will increase. _________________ Frank |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Sat Jun 05, 2010 8:21 am Post subject: |
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digital wrote: | As the tyre wears, [SNIP}The difference between true speed (as indicated by TT) and the speedo will increase. The error will increase. | By about 2% (again if my sums are right) |
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digital Regular Visitor
Joined: Nov 29, 2004 Posts: 88 Location: Leeds, UK
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Posted: Sat Jun 05, 2010 9:59 am Post subject: |
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M8TJT wrote: | digital wrote: | As the tyre wears, [SNIP}The difference between true speed (as indicated by TT) and the speedo will increase. The error will increase. | By about 2% (again if my sums are right) |
Thanks. So the speedo, which is already an illegal 11% out at 70mph (TT true speed 72, car indicated 80), will be even more illegal (if there is such a concept) at 13%.
I think I'll see what Vauxhall have to say about recalibration. _________________ Frank |
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Robin2 Lifetime Member
Joined: Nov 24, 2003 Posts: 1441 Location: Swansea
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Posted: Sat Jun 05, 2010 10:19 am Post subject: |
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Digital, yours seems more than usually inaccurate - the usual error is typically about 5% fast (though as others have said, it can legally be anything between 0 and 10% fast. I'm sure you have read this in the link which was posted, but you can only rely on the GPS speed if you are driving at a steady speed, on the level, in a straight line.
For the record, for a 17 inch wheel, a loss of 6mm of rubber can make a difference to your speedo accuracy of just over 2.5%
You probably have a case to pursue with Vauxhall
Robin _________________ TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
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Plus a lot of other PDA GPS kit, seldom used |
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digital Regular Visitor
Joined: Nov 29, 2004 Posts: 88 Location: Leeds, UK
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Posted: Sat Jun 05, 2010 10:26 am Post subject: |
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Thanks.
Yes, I had read all the health warnings and checked it in the appropriate circumstances!
I was also aware of the 10% rule, which is why I decided to check it out carefully. I had noticed that when I was driving through stretches of road covered by specs I was noticeably slower than a lot of the other traffic whereas in my previous car I was well up with them.
Being so far out, I'm wondering if Vauxhall have calibrated it on a different size of wheel and not corrected for those fitted to my model. _________________ Frank |
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Anita Pocket GPS Moderator
Joined: Mar 15, 2006 Posts: 3219 Location: Windlesham, Surrey
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Posted: Sat Jun 05, 2010 10:32 am Post subject: |
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Robin2 wrote: | . . . you can only rely on the GPS speed if you are driving at a steady speed, on the level, in a straight line. |
The calculations have been posted in a previous thread and, even when those conditions do not apply, the GPS is likely to show a considerably more accurate speed than the car's speedo. _________________ Anita
TomTom VIA 135 - App 12.075
UK map 1130.12368
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johnm52 Occasional Visitor
Joined: Sep 11, 2005 Posts: 31
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Posted: Sat Jun 05, 2010 8:48 pm Post subject: |
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I am running a three week old Astra as a hire car this weekend. Speed check today showed 71 on TomTom as 79 on both car speedo and repeater digital display on dash. Also 40 on TomTom showed 44/45 on both speedo and repeater display. This would seem consistent with OP. |
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