Accuracy of GPS Speed indicators

jaykay2 · Posted: Sat Aug 20, 2005 11:59 am Post subject:

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

mcintyre · Posted: Sun Oct 30, 2005 9:10 am Post subject:

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....

Bigstoo · Posted: Wed Nov 09, 2005 12:22 am Post subject:

private test track? M1 per chance? 8)

gune · Posted: Sun Nov 13, 2005 2:57 am Post subject: acer n35 destinator

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?
Exclamation

neil01 · Posted: Sun Nov 13, 2005 2:11 pm Post subject:

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!

Ako · Posted: Tue Nov 15, 2005 9:40 pm Post subject:

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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Holux231, XDAII Copilot 6, 1Gb Lexar SD Card

peterc10 · Posted: Tue Nov 15, 2005 11:54 pm Post subject:

MikeProcter · Posted: Wed Nov 23, 2005 12:48 am Post subject:

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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Skoda Columbus and for emergencies: TomTom + iPhone 4 & TomTom car kit

jguiver · Posted: Thu Nov 24, 2005 10:28 am Post subject:

(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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HP rx5935, 2GB SD Card, TTN 6.1, GPSDash

neil01 · Posted: Thu Nov 24, 2005 3:19 pm Post subject:

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?

jguiver · Posted: Thu Nov 24, 2005 5:51 pm Post subject:

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

MikeProcter · Posted: Thu Nov 24, 2005 7:03 pm Post subject:

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

neil01 · Posted: Thu Nov 24, 2005 7:17 pm Post subject:

jguiver · Posted: Thu Nov 24, 2005 7:55 pm Post subject:

neil01 · Posted: Fri Nov 25, 2005 12:08 am Post subject:

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

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