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jaykay2 Occasional Visitor
Joined: Aug 08, 2005 Posts: 6 Location: west yorkshire
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Posted: Sat Aug 20, 2005 11:59 am Post subject: |
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For years i sold dynamometers to the motor trade, it was generally accepted that car speedo's were 10 percent high so at 70MPH real speed (verified by the dyno) the car's speedo would read 77, by that same maths the error at a lower speed, say 30MPH is only 3,
this is certainly borne out by my tests with my GPS unit on a level road at a constant speed.
hope this helps
John Kilcline
Yakumo Delta X with Tomtom 5 |
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mcintyre Occasional Visitor
Joined: Oct 30, 2005 Posts: 49 Location: Mirfield
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Posted: Sun Oct 30, 2005 9:10 am Post subject: |
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I recently noticed this too, as I have just driven the length of England - where conditions allowed we (there were a group of us) did some testing (read - playing) and I found that when my car speedo shows 70 the GPS only shows 66 and when my car speedo is showing 100 the GPS says I'm only doing 92.
All speeds quoted on a private test track of course.... |
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Bigstoo Occasional Visitor
Joined: Nov 01, 2005 Posts: 29 Location: Republic of Yorkshire
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Posted: Wed Nov 09, 2005 12:22 am Post subject: |
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private test track? M1 per chance? 8) |
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gune Regular Visitor
Joined: Mar 15, 2005 Posts: 174 Location: Colchester, UK
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Posted: Sun Nov 13, 2005 2:57 am Post subject: acer n35 destinator |
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hi
i found that most of the time my acer read 10mph below the speedo.
also the legal limit for paritcular road at times.
if the notice says 50mph, pda will start warning at 40mph..saying ur over the speed limit..
ofcourse so i go by speedo..
u cant argue in court saying my gps is foul?
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neil01 Frequent Visitor
Joined: May 06, 2005 Posts: 902 Location: Leeds
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Posted: Sun Nov 13, 2005 2:11 pm Post subject: |
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I think that you will find that, not only can't you argue that your speedo was to blame, but if you tried you might even get an additional conviction for a faulty speedo! |
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Ako Regular Visitor
Joined: Oct 28, 2004 Posts: 128 Location: Cheshire
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Posted: Tue Nov 15, 2005 9:40 pm Post subject: |
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The requirement in the uk is for a speedo to be within 10%. Now into the realms of fantasy:- I'm told that the point at which you would normally be caught by our law enforcement guys is at 10% + 2. I'm also told that speedo's are artificially set high. Now I'm willing to be shot down in flames here but I've not heard of anyone being convicted within the 10% accuracy.
Dave _________________ Holux231, XDAII Copilot 6, 1Gb Lexar SD Card |
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peterc10 Frequent Visitor
Joined: Aug 21, 2005 Posts: 1761 Location: Kent, England
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Posted: Tue Nov 15, 2005 11:54 pm Post subject: |
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Ako wrote: | The requirement in the uk is for a speedo to be within 10%. Now into the realms of fantasy:- I'm told that the point at which you would normally be caught by our law enforcement guys is at 10% + 2. I'm also told that speedo's are artificially set high. Now I'm willing to be shot down in flames here but I've not heard of anyone being convicted within the 10% accuracy.
Dave |
Speedos are not set artificially high but all they do is count the revolutions of a wheel per minute and convert that to speed. But that conversion relies upon knowing the diameter of the wheel and tyre. the larger the diameter the further the car travels per revolution. The diameter depends upon car version (some have larger wheels than others and yet the speedo is the same), tyre make, pressure, and wear. So in order to make sure that they do not produce speedo that reads too low they calculate on the largest possible diameter. Hence most speedos read too high. |
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MikeProcter Regular Visitor
Joined: Sep 24, 2004 Posts: 220 Location: Croydon, United Kingdom
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Posted: Wed Nov 23, 2005 12:48 am Post subject: |
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and why is is that many drivers for instance on say the big carpark around London - (NCP call it the M25!) slam on their brakes to reduce their speed to say 35-38mph on the speedo whilst going past the safety(?) cameras? I don't personally call that safe, causing us law abiding citizens to have to brake to avoid hitting the car in front who has just slowed down & pulled in front of you........& all I was doing officer was 40mph according to the gospel of Tom Tom. _________________ Skoda Columbus and for emergencies: TomTom + iPhone 4 & TomTom car kit |
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jguiver Regular Visitor
Joined: Jul 26, 2005 Posts: 65 Location: No valid GPS Signal!
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Posted: Thu Nov 24, 2005 10:28 am Post subject: |
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(Anorak mode on)
I've tried to check the speedometers in previous cars for accuracy either by timing on a motorway between markers, or using the rev counter and the speed per 1000 rpm in top gear. If these calculations were anywhere near accurate, then all my speedos have read fast to varying degrees, usually by about 10%.
This is the first car I've had where I have also had GPS, and on a level straight road at a constant speed with correctly inflated newish tyres, the speedo appears to be spot on, if Tomtom is to be believed.
(Anorak mode off) _________________ HP rx5935, 2GB SD Card, TTN 6.1, GPSDash |
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neil01 Frequent Visitor
Joined: May 06, 2005 Posts: 902 Location: Leeds
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Posted: Thu Nov 24, 2005 3:19 pm Post subject: |
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Unfortunately your anorak may have a hole in it! Unless you have had it confirmed, how do you know that your rev counter is itself correct? |
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jguiver Regular Visitor
Joined: Jul 26, 2005 Posts: 65 Location: No valid GPS Signal!
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Posted: Thu Nov 24, 2005 5:51 pm Post subject: |
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I don't. Hence the 'if'.
My point was that my current speedo seems accurate as it indicates the same as my GPS. Prior to this I only had two ways of trying to check the accuracy. Well, three actually, although the third would involve getting a speeding ticket! _________________ HP rx5935, 2GB SD Card, TTN 6.1, GPSDash |
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MikeProcter Regular Visitor
Joined: Sep 24, 2004 Posts: 220 Location: Croydon, United Kingdom
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Posted: Thu Nov 24, 2005 7:03 pm Post subject: |
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I drove a few hundred miles today and noticed that my BMW speedo is over by 2-3 mph according to Tom Tom at speeds between 40-80mph indicated. Towns were too wet & dangerous to do a check at 30-40mph. _________________ Skoda Columbus and for emergencies: TomTom + iPhone 4 & TomTom car kit |
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neil01 Frequent Visitor
Joined: May 06, 2005 Posts: 902 Location: Leeds
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Posted: Thu Nov 24, 2005 7:17 pm Post subject: |
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jguiver wrote: | I don't. Hence the 'if'.
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When saying that your anorak MIGHT have a hole in it, I forgot to mention about my cheek having a tongue well and truly in it! |
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jguiver Regular Visitor
Joined: Jul 26, 2005 Posts: 65 Location: No valid GPS Signal!
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Posted: Thu Nov 24, 2005 7:55 pm Post subject: |
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neil01 wrote: | jguiver wrote: | I don't. Hence the 'if'.
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When saying that your anorak MIGHT have a hole in it, I forgot to mention about my cheek having a tongue well and truly in it! |
Sorry - re-reading my post, it sounds a little brusque. No offence intended!
I suppose a speedo is likely to become more optimistic as the tyres wear & their circumference becomes smaller. I'll check again in another 10,000 miles................ _________________ HP rx5935, 2GB SD Card, TTN 6.1, GPSDash |
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neil01 Frequent Visitor
Joined: May 06, 2005 Posts: 902 Location: Leeds
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Posted: Fri Nov 25, 2005 12:08 am Post subject: |
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None taken, my tongue was still stuck in my cheek! |
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