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Accuracy of GPS Speed indicators
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jaykay2
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Joined: Aug 08, 2005
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Location: west yorkshire

PostPosted: Sat Aug 20, 2005 11:59 am    Post subject: Reply with quote

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
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mcintyre
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Location: Mirfield

PostPosted: Sun Oct 30, 2005 9:10 am    Post subject: Reply with quote

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
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PostPosted: Wed Nov 09, 2005 12:22 am    Post subject: Reply with quote

Laughing Out Loud private test track? M1 per chance? 8)
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gune
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Joined: Mar 15, 2005
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Location: Colchester, UK

PostPosted: Sun Nov 13, 2005 2:57 am    Post subject: acer n35 destinator Reply with quote

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.. Confused
ofcourse so i go by speedo..
u cant argue in court saying my gps is foul?
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neil01
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PostPosted: Sun Nov 13, 2005 2:11 pm    Post subject: Reply with quote

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
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PostPosted: Tue Nov 15, 2005 9:40 pm    Post subject: Reply with quote

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
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peterc10
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PostPosted: Tue Nov 15, 2005 11:54 pm    Post subject: Reply with quote

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
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PostPosted: Wed Nov 23, 2005 12:48 am    Post subject: Reply with quote

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.
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jguiver
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PostPosted: Thu Nov 24, 2005 10:28 am    Post subject: Reply with quote

(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)
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neil01
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PostPosted: Thu Nov 24, 2005 3:19 pm    Post subject: Reply with quote

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
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PostPosted: Thu Nov 24, 2005 5:51 pm    Post subject: Reply with quote

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!
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MikeProcter
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PostPosted: Thu Nov 24, 2005 7:03 pm    Post subject: Reply with quote

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.
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neil01
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PostPosted: Thu Nov 24, 2005 7:17 pm    Post subject: Reply with quote

jguiver wrote:
I don't. Hence the 'if'.



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
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PostPosted: Thu Nov 24, 2005 7:55 pm    Post subject: Reply with quote

neil01 wrote:
jguiver wrote:
I don't. Hence the 'if'.



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. Embarassed 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................
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neil01
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PostPosted: Fri Nov 25, 2005 12:08 am    Post subject: Reply with quote

None taken, my tongue was still stuck in my cheek!
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