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KathS
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Location: Nr Bedford
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 Posted: Fri Nov 13, 2009 10:04 am Post subject: Speedo vs gps |
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This must have been asked before but i can't find it!
I've noticed that the speed on my cars speedo (Ford fiesta) always reads about 3mph faster than the speed recorded on my Drivesmart (a speed camera device which uses gps). (e.g. when my Drivemart says I'm doing 30 the speedo says 33)
Which is most accurate and why?
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BigPerk
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| Posted: Fri Nov 13, 2009 10:20 am Post subject: |
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The GPS is the more accurate one (with normal reception). Car's speedos always read fast - I think there is some legal point indicating that they should be made deliberately fast by up to 10%.
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jolly47roger
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| Posted: Fri Nov 13, 2009 10:20 am Post subject: |
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| As long as the GPS has a reasonably clear view of the sky, it will be more accurate. Car manufacturers err on the high side to reduce the risk of inadvertent speeding. |
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KathS
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Location: Nr Bedford
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| Posted: Fri Nov 13, 2009 10:52 am Post subject: |
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| Great, thanks! |
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M8TJT
The Other Tired Old Man
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Location: Bexhill, South Sussex, UK
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| Posted: Fri Nov 13, 2009 10:55 am Post subject: |
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SIRF STAR lll Spec.
Horizontal Position Accuracy
Autonomous <2.5 m (8.2ft)
SBAS <2.0 m
Velocity Accuracy
Speed <0.01 m/s
Heading <0.01°
(SBAS = Satellite Based Augmentation System )
Note that the speed accuracy is about 0.022MPH
De-rating this by a factor of 10 for various sat lock errors etc. still gives an accuracy of 0.2MPH. I think that this is probably a bit more accurate than your speedo which is set by the manufacturers to over-read for the reasons given in earlier posts. |
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ajbristol
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Joined: Aug 28, 2007
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| Posted: Tue Dec 01, 2009 11:27 pm Post subject: Speedo vs GPS |
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I think you'll probably survive a tug by the boys in blue as MOST forces operate a policy of (speed)+10%+2 so say a 30MPH road you would have to be in excess of 35mph before you would get a ticket UNLESS your very unlucky and catch someone on a bad day or they need to make up the numbers, (near the end of the month normally).
Its easier just to drive within the speed limits, remembering that ??mph is the maximum speed not a target ! |
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KathS
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Location: Nr Bedford
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| Posted: Wed Dec 02, 2009 12:30 am Post subject: |
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| I wasn't asking so that I could break the limit! |
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DennisN
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Joined: Feb 27, 2006
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| Posted: Wed Dec 02, 2009 12:32 am Post subject: |
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| M8TJT wrote: | SIRF STAR lll Spec.
Horizontal Position Accuracy
Autonomous <2.5 m (8.2ft)
SBAS <2.0 m
Velocity Accuracy
Speed <0.01 m/s
Heading <0.01°
(SBAS = Satellite Based Augmentation System )
Note that the speed accuracy is about 0.022MPH
De-rating this by a factor of 10 for various sat lock errors etc. still gives an accuracy of 0.2MPH. |
What the ??????????????? Lord help us poor stupids! 
The way I look at it is - the satnav calculates your speed by timing how long it takes you to move from one point to another. Now I am far too thick to have any way of showing it in small distances, so I simply think of it as if you travel one mile in one minute, you're doing 60mph. Only the satnav does the calculations every time it refreshes - rather more often than a mile apart - yards indeed. If you've travelled 15 yards in one second, I haven't the faintest idea how fast you're going, but I'll bet M8TJT can tell you and that's how the satnav is more accurate.
Your car speedo only works off the wheels or gearbox, or somesuch mechanical thing and even something as simple as if your tyres are hard or soft, they'll affect the true speed. 15 yards on your car tyres in one second will be similar, but not accurate due to tyre pressures, tyre wear etc. The law says you can have a speedo which shows the speed faster than true, but it must NOT show a speed which is not as fast as true. So manufacturers deliberately make them read a speed faster than true - effectively, the speedo may say you're doing maybe 30mph when you're really only doing 28mph.
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If it tastes good - it's fattening.
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ajbristol
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| Posted: Wed Dec 02, 2009 12:54 am Post subject: speedo vs gps |
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I wasn't implying you were doing it to speed, my point was and in answer to your original question which and why....
which : would be the gps most accurate but not exact
why : because you drive a car which does not have a calibrated speedo |
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M8TJT
The Other Tired Old Man
Joined: Apr 04, 2006
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Location: Bexhill, South Sussex, UK
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| Posted: Wed Dec 02, 2009 8:49 am Post subject: |
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| DennisN wrote: | What the ??????????????? Lord help us poor stupids!
The way I look at it is - the satnav calculates your speed by timing how long it takes you to move from one point to another. Now I am far too thick to have any way of showing it in small distances, so I simply think of it as if you travel one mile in one minute, you're doing 60mph. Only the satnav does the calculations every time it refreshes - rather more often than a mile apart - yards indeed. If you've travelled 15 yards in one second, I haven't the faintest idea how fast you're going, but I'll bet M8TJT can tell you and that's how the satnav is more accurate |
30.682MPH , but no, they don't do thedistance /time sum, simplistically, they use the doppler shift on the received gps signals. So there.  You did ask 
I expect someone else will come along and explain excactly how, or you could Google for it if you are that interested. Needless to say, the GPS will, under good signal conditions, be considerably more accurate than the old electro mechanical speedo. |
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KathS
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Joined: May 30, 2007
Posts: 24
Location: Nr Bedford
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| Posted: Wed Dec 02, 2009 9:09 am Post subject: |
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Thanks everyone your replies are really interesting, it's always good to know how stuff works and when I saw two different measurements of my speed I was just curious to know which was most likely to be right!
ajbristol: I was being too sensitive, sorry!  |
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