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nivek22 Lifetime Member
Joined: Mar 09, 2006 Posts: 194 Location: Barnsley, South Yorkshire.
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Posted: Tue Jan 15, 2008 9:34 am Post subject: |
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tgold wrote: | I am surprised that a vehicle technician believes inflation pressure affects the rolling circumference of a tyre or the accuracy of the speedometer. Anyway, I look forward to reading your results.
Tony |
If you take this to extremes and consider a flat tyre. The diameter has reduced by about 6 inches, so the circumference will have reduced proportionately. (Can't be bothered to work it out). |
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Snudge Lifetime Member
Joined: Aug 22, 2007 Posts: 211 Location: Peterborough
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Posted: Tue Jan 15, 2008 10:51 am Post subject: |
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nivek22 wrote: | If you take this to extremes and consider a flat tyre. The diameter has reduced by about 6 inches, so the circumference will have reduced proportionately. (Can't be bothered to work it out). |
1)Take a piece of string 1 metre long and make it into a circle to represent an inflated tyre and measure the circumference. (It's got to be 1 metre.)
2)Now make it into the shape of a flat tyre and measure the circumference.
Is there a difference?
( I think the thread has gone the same way as the last one on this subject - sorry for contributing ) |
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StanStill Regular Visitor
Joined: Feb 27, 2005 Posts: 81
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Posted: Tue Jan 15, 2008 11:15 am Post subject: |
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What nobody has mentioned is that tyre wear plays a part in the accuracy of your speedo reading. A new tyre has a greater circumference than a worn down one. I think this is one of the reasons that manufactures have to build in a bit of slack in their speedo readings. |
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if1977 Lifetime Member
Joined: Nov 30, 2007 Posts: 649 Location: Doncaster
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Posted: Tue Jan 15, 2008 1:46 pm Post subject: |
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OK, Here goes.
I took a brand new car out for a test drive about an hour ago and it was a mid-range 5-door with 195-55-15 tyres on it (alloy wheel 5 stud). I did as i said in the previous post and the findings were. Just for re-assurance another college was driving so i could consentrate in the screens.
At 30mph (According to the cars speedo) the actual speed of the car(according to the diagnostic machine in parameters) was 28mph and the speed according to the satnav was 33-34 mph(changed between these about 4 times in a 20 second stretch). We done the same test at 70mph (according to the cars speedo) the actual speed (according to the diagnostic machine) was 66mph and the speed according to the satnav was 75 mph. These were carried out when it was raining and on straight(ish) roads and at a constant speed for 20-30 seconds.
A bit of technical jargon.
On the same make of car wether it has 13'', 14'', 15'', 16'' wheels and does not matter what size of tyre it is the speedo drive( which is in the gearbox and measures the speed of the cars drive shafts) then sends the information to the cars ECU ( Electronic Control Unit) which then adjusts the speedo reading to legal % adjustment for the road and then that reading is the one that gets sent to the speedo head in the car which you see. So this is why if you change the wheel or tyre size of the car it will change the revolutions of the driveshart which will give the ECU a different reading and that will give the speedo head a wrong reading also.
Quote: | tgold wrote:
I am surprised that a vehicle technician believes inflation pressure affects the rolling circumference of a tyre or the accuracy of the speedometer. Anyway, I look forward to reading your results.
Tony
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It depends on how flat or hard the tyre is because if the tyre is over inflated and it is a slippy road then this will cause the tyre to spin more than the correct pressure and give you a faster reading than you are actually traveling or if it is about 5-10 psi (Flat) this will cause the tyre to broaden and that will put the car closer to the road and will cause drag which also could alter slightly(probably wont notice) and will also put ther car under load and burn more fuel.
I would also like to say that my Astra and TT do not agree by a long shot. When the TT says i am doing 70mph my car speedo says i am doing about 78mph and my Astra is the way it came from the factory. How do they explain this.
As i say these are the readings i saw with my own eyes and the technical jargon came out of a franchise manual for that spesific car so i cannot disagree with it. And if you find your readings between your speedo and satnav are large then maybe the wheels on that car were not standard from the factory.
If you have any questions i will answer them as best as i can and if you would like to know more about it just ask. _________________ NEW Tomtom GO 500 (hope it is good)
iphone (running all day, doesn't like it)
XL LIVE IQ Routes (Got stolen and miss it)
GO 510 (in drawer now, time for upgrade) |
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jspencer Occasional Visitor
Joined: Dec 06, 2007 Posts: 30 Location: Brighton, MA, USA
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Posted: Tue Jan 15, 2008 3:28 pm Post subject: |
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How does the diagnostic machine arrive at its readings?
Jon |
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Robin2 Lifetime Member
Joined: Nov 24, 2003 Posts: 1441 Location: Swansea
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Posted: Tue Jan 15, 2008 4:28 pm Post subject: |
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if1977 wrote: |
I would also like to say that my Astra and TT do not agree by a long shot. When the TT says i am doing 70mph my car speedo says i am doing about 78mph and my Astra is the way it came from the factory. How do they explain this.
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If the TT speed is 70 mph (+/- 1 mph) and the Astra's speedo says 77 mph it is (just) within tolerance, which is +7/-0 mph at 70)
The higher speeds shown by satnav in if1977's comparative test are completely at variance with my own experience in many cars, and also results quoted in this forum by a large number of other contributors.
Given that the results obtained by the diagnostic machine are about what I would expect, I can only conclude that the satnav used in the test was defective
Robin _________________ TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used |
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if1977 Lifetime Member
Joined: Nov 30, 2007 Posts: 649 Location: Doncaster
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Posted: Tue Jan 15, 2008 5:12 pm Post subject: |
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I did not do the test in my Astra Robin2. I just added that at the end to tell people what my car was like. My Astra is a 1998 model and has about 80k on it so it is not the newest car and will have some wear and tear to it and would say my satnav is OK because it has been checked with others. _________________ NEW Tomtom GO 500 (hope it is good)
iphone (running all day, doesn't like it)
XL LIVE IQ Routes (Got stolen and miss it)
GO 510 (in drawer now, time for upgrade) |
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mikealder Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Tue Jan 15, 2008 5:54 pm Post subject: |
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Ian, from what you describe the car you took for a test drive is illegal, I would not expect the GPS speed to be inaccurate enough to explain the difference you observed, had the car been modified in way by the garage - Mike |
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if1977 Lifetime Member
Joined: Nov 30, 2007 Posts: 649 Location: Doncaster
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Posted: Tue Jan 15, 2008 6:39 pm Post subject: |
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mikealder wrote: | Ian, from what you describe the car you took for a test drive is illegal, I would not expect the GPS speed to be inaccurate enough to explain the difference you observed, had the car been modified in way by the garage - Mike |
It was a new car that had been delivered in a batch of about 10 and was checked for the road and had a CD multiplayer fitted and mudflaps and floor mats, otherwise it is the way it came off the lorry with a delivery milage of 000007 miles on the clock.
Why do you think this innacuracy would make the car illegal or altered in any way. _________________ NEW Tomtom GO 500 (hope it is good)
iphone (running all day, doesn't like it)
XL LIVE IQ Routes (Got stolen and miss it)
GO 510 (in drawer now, time for upgrade) |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Jan 15, 2008 6:51 pm Post subject: |
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mikealder wrote: | I wonder if this thread will go the same wandering path the last one did that related to speedo accuracy? |
Looks like...... |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Jan 15, 2008 6:55 pm Post subject: |
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Snudge wrote: |
1)Take a piece of string 1 metre long and make it into a circle to represent an inflated tyre and measure the circumference. (It's got to be 1 metre.)
2)Now make it into the shape of a flat tyre and measure the circumference.
Is there a difference? |
Obviously not, but that's nothing to do with the situation here.
what IS relevant is the effective radius of the inflated or deflated tyres.
When running on a deflated tyre the distance between the centre of the wheel and the road is smaller.
Imagine the extreme example of no tyre - running on the rims, and that certainly means a smaller circumference, doesn't it. |
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tgold Regular Visitor
Joined: Nov 07, 2004 Posts: 98 Location: UK or USA from time to time.
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Posted: Tue Jan 15, 2008 7:00 pm Post subject: |
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Robin2 wrote: | If the TT speed is 70 mph (+/- 1 mph) and the Astra's speedo says 77 mph it is (just) within tolerance, which is +7/-0 mph at 70)
| As I understand it the legal tolerance is +0 and - (10% plus 10kph). So if a new vehicle travels at 70 mph and its speedometer reads anything between around 56.75 mph and 70 mph it would meet the legal requirements under the Construction and Use Regulation, now based on Community Directive 97/39.
Tony |
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tgold Regular Visitor
Joined: Nov 07, 2004 Posts: 98 Location: UK or USA from time to time.
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Posted: Tue Jan 15, 2008 7:05 pm Post subject: |
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Andy_P2002 wrote: | what IS relevant is the effective radius of the inflated or deflated tyres.
When running on a deflated tyre the distance between the centre of the wheel and the road is smaller.
Imagine the extreme example of no tyre - running on the rims, and that certainly means a smaller circumference, doesn't it. | Maybe forget "radius" for a non-circular tyre and consider just the perimeter, called the rolling circumference. And when considering it, imagine it is made from steel belting which is virtually inelastic. Does that change the picture? Obviously if you remove the tyre and run on the rims, that is not an extreme example of deflation but quite a different situation altogether.
Tony |
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mikealder Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Tue Jan 15, 2008 7:07 pm Post subject: |
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With a GPS speed of 70 mph I would expect the vehicles speedo to read between 70 to 77. What is not acceptable is for the vehicles speedo to read under the actual speed the vehicle is travelling at, so where you had a speedo reading of 70MPH and the GPS speed was indicating 75 the speedo is illegal - it is enticing you to break the law - Mike |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Jan 15, 2008 7:17 pm Post subject: |
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tgold wrote: | Maybe forget "radius" for a non-circular tyre and consider just the perimeter, called the rolling circumference. And when considering it, imagine it is made from steel belting which is virtually inelastic. Does that change the picture? Obviously if you remove the tyre and run on the rims, that is not an extreme example of deflation but quite a different situation altogether.
Tony |
Nope, I still disagree. When the tyre is totally deflated, isn't that virtually the same as running on the rims? The car is certainly nearer the ground, that's why they deflate the tyres on trucks to help get them under low bridges.
So (I think) the effective circumference is defined by the new (smaller) radius.
It's a good puzzle though... I'm going to ask our resident work genius about it tomorrow. |
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