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PW
Occasional Visitor
Joined: Apr 13, 2004
Posts: 40
Location: Edinburgh
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 Posted: Thu Dec 02, 2004 5:44 pm Post subject: |
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| uffe73 wrote: | Can anyone tell me what the best theoretical speed accuracy is in a GPS based navigation system when you're driving at constant speed and constant height above sea level on a straight road?
/Ulf |
I would say the best theoretical (and practical) accuracy is 100%.
That would occur when you are stationary 
Ok.... I don't know the real answer
Pete |
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dibble
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Joined: Nov 18, 2004
Posts: 28
Location: Northants - U.K
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| Posted: Tue Dec 07, 2004 10:39 pm Post subject: |
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I did the very same check tonight on my way home. 70mph on the speedo and TT3 telling me 67mph. seems quite consistant with most on this thread.
Like your style PW!! 
Dibs |
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Robin2
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Joined: Nov 24, 2003
Posts: 1441
Location: Swansea
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| Posted: Wed Dec 08, 2004 12:50 am Post subject: |
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There isn't a single answer. As has been said several times on this and other threads, by law vehicles sold in the UK must have speedos which read between 0 and 10% fast. It's illegal to sell a car which underestimates speed. Typical values are about 3 or 4% fast - in my experience, Fords seem to overestimate speed by the greatest margin
Robin |
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psyskiesman
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Joined: Jun 22, 2004
Posts: 322
Location: Swindon, Wiltshire
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| Posted: Wed Dec 08, 2004 7:22 pm Post subject: TT3 speed accuracy |
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| I am in a fortunate position in my job. I deliver new cars, mostly Fords, to private and public sector clients. This latter category extends to police forces and occasionally a car I am delivering will have an accurately calibrated speedo. When cruise control is set to 70mph on the speedo, my satnav (2210, Fortuna clip-on, TT3) indicates exactly 70! Likewise for 45mph, 60mph and 79 mph, the highest speed I ever drive. By and large most speedos indicate about 4 or 5 mph more than the actual speed. |
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kragbell
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Joined: Dec 09, 2004
Posts: 13
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| Posted: Thu Dec 09, 2004 3:47 pm Post subject: |
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| MarkHewitt wrote: | .....
This is very useful information, especially in these days of speed cameras etc. Being able to that extra 5mph legally can make quite a difference to your progress over long distances. |
I noticed the speed difference (again about 6-10%) on a journey and it got me thinking that, if I continue to progress using my vehicle speedo (much easier and faster to read than tomtom - but the tomtom is acceptable) then the accuracy of tomtom's Estimated Time of Arrival is diminished. I can, for free, manipulate the average speeds in tomtom to reflect the 8% increase in vehicle speedo reading or (at a probably significant cost) re-calibrate my speedo in the car (possibly). Any comments ? |
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Robin2
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Joined: Nov 24, 2003
Posts: 1441
Location: Swansea
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| Posted: Thu Dec 09, 2004 4:39 pm Post subject: |
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I don't understand the logic. If you can determine experimentally that your speedo is, say, 10% fast at 70mph, why not just use your car speedo and drive at an indicated 77 mph on motorways?
Robin |
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kragbell
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Joined: Dec 09, 2004
Posts: 13
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| Posted: Thu Dec 09, 2004 5:27 pm Post subject: |
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| Robin2 wrote: | I don't understand the logic. If you can determine experimentally that your speedo is, say, 10% fast at 70mph, why not just use your car speedo and drive at an indicated 77 mph on motorways?
Robin |
I agree that can be done but I'd rather the speedo read correctly (although if it's going to cost more than a tenner to get it calibrated....)
Plus - I can see my wife's face already when I try to justify my doing over 70 based on the relative merits of speeds calculated using satellites miles in the air compared to the speedo straight in front of my face. Me shudders. |
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Thu Dec 09, 2004 7:20 pm Post subject: |
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| kragbell wrote: | | if I continue to progress using my vehicle speedo then the accuracy of tomtom's Estimated Time of Arrival is diminished. |
I don't have experience with TomTom specifically, but Garmin and CoPilot Sat Nav systems seem to dynamically adjust the ETA based on your actual average speed.
I have seen this happen when travelling on a motorway in heavy traffic at say 40 MPH for a while then you speed up to 70 MPH ( ) when the traffic clears and you can see the ETA count backwards as your average speed increases.
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