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TT 930 speed accuracy.

 
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digital
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Location: Leeds, UK

PostPosted: Fri Jun 04, 2010 11:54 pm    Post subject: TT 930 speed accuracy. Reply with quote

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
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Anita
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PostPosted: Sat Jun 05, 2010 12:21 am    Post subject: Reply with quote

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.
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digital
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PostPosted: Sat Jun 05, 2010 12:36 am    Post subject: Reply with quote

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.
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Anita
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PostPosted: Sat Jun 05, 2010 12:47 am    Post subject: Reply with quote

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. Smile
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mikealder
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PostPosted: Sat Jun 05, 2010 12:57 am    Post subject: Reply with quote

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
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PostPosted: Sat Jun 05, 2010 1:22 am    Post subject: Reply with quote

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.
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M8TJT
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PostPosted: Sat Jun 05, 2010 8:21 am    Post subject: Reply with quote

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
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PostPosted: Sat Jun 05, 2010 9:59 am    Post subject: Reply with quote

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.
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Robin2
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PostPosted: Sat Jun 05, 2010 10:19 am    Post subject: Reply with quote

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
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digital
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PostPosted: Sat Jun 05, 2010 10:26 am    Post subject: Reply with quote

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.
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Anita
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PostPosted: Sat Jun 05, 2010 10:32 am    Post subject: Reply with quote

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.
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johnm52
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PostPosted: Sat Jun 05, 2010 8:48 pm    Post subject: Reply with quote

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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