Home PageFacebookRSS News Feed
PocketGPS
Web
SatNav,GPS,Navigation
Pocket GPS World - SatNavs | GPS | Speed Cameras: Forums

Pocket GPS World :: View topic - Speedo vs gps
 Forum FAQForum FAQ   SearchSearch   UsergroupsUsergroups   ProfileProfile   Log in for private messagesLog in for private messages   Log inLog in 

Speedo vs gps

 
Post new topic   Reply to topic    Pocket GPS World Forum Index -> Beginners GPS Lounge
View previous topic :: View next topic  
Author Message
KathS
Occasional Visitor


Joined: May 30, 2007
Posts: 24
Location: Nr Bedford

PostPosted: Fri Nov 13, 2009 10:04 am    Post subject: Speedo vs gps Reply with quote

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?

Kath
Back to top
View user's profile Send private message
BigPerk
Frequent Visitor


Joined: Sep 06, 2006
Posts: 1618
Location: East Hertfordshire

PostPosted: Fri Nov 13, 2009 10:20 am    Post subject: Reply with quote

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%.
_________________
David
(Navigon 70 Live, Nuvi 360)
Back to top
View user's profile Send private message
jolly47roger
Regular Visitor


Joined: Jan 11, 2008
Posts: 116

PostPosted: Fri Nov 13, 2009 10:20 am    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
KathS
Occasional Visitor


Joined: May 30, 2007
Posts: 24
Location: Nr Bedford

PostPosted: Fri Nov 13, 2009 10:52 am    Post subject: Reply with quote

Great, thanks!
Back to top
View user's profile Send private message
M8TJT
The Other Tired Old Man
The Other Tired Old Man


Joined: Apr 04, 2006
Posts: 10118
Location: Bexhill, South Sussex, UK

PostPosted: Fri Nov 13, 2009 10:55 am    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
ajbristol
Occasional Visitor


Joined: Aug 28, 2007
Posts: 4

PostPosted: Tue Dec 01, 2009 11:27 pm    Post subject: Speedo vs GPS Reply with quote

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 !
Back to top
View user's profile Send private message
KathS
Occasional Visitor


Joined: May 30, 2007
Posts: 24
Location: Nr Bedford

PostPosted: Wed Dec 02, 2009 12:30 am    Post subject: Reply with quote

I wasn't asking so that I could break the limit!
Back to top
View user's profile Send private message
DennisN
Tired Old Man
Tired Old Man


Joined: Feb 27, 2006
Posts: 14886
Location: Keynsham

PostPosted: Wed Dec 02, 2009 12:32 am    Post subject: Reply with quote

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! Shocked

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.
_________________
Dennis

If it tastes good - it's fattening.

Two of them are obesiting!!
Back to top
View user's profile Send private message
ajbristol
Occasional Visitor


Joined: Aug 28, 2007
Posts: 4

PostPosted: Wed Dec 02, 2009 12:54 am    Post subject: speedo vs gps Reply with quote

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
Back to top
View user's profile Send private message
M8TJT
The Other Tired Old Man
The Other Tired Old Man


Joined: Apr 04, 2006
Posts: 10118
Location: Bexhill, South Sussex, UK

PostPosted: Wed Dec 02, 2009 8:49 am    Post subject: Reply with quote

DennisN wrote:
What the ??????????????? Lord help us poor stupids! Shocked

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. Twisted Evil You did ask Very Happy
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.
Back to top
View user's profile Send private message
KathS
Occasional Visitor


Joined: May 30, 2007
Posts: 24
Location: Nr Bedford

PostPosted: Wed Dec 02, 2009 9:09 am    Post subject: Reply with quote

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! Smile
Back to top
View user's profile Send private message







Posted: Today    Post subject: Pocket GPS Advertising

Back to top
Display posts from previous:   
Post new topic   Reply to topic    Pocket GPS World Forum Index -> Beginners GPS Lounge All times are GMT + 1 Hour
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum

Make a Donation



CamerAlert Database

Click here for the PocketGPSWorld.com Speed Camera Database

Download Speed Camera Database
21.094 (27 Sep 23)



WORLDWIDE SPEED CAMERA SPOTTERS WANTED!

Click here to submit camera positions to the PocketGPSWorld.com Speed Camera Database


12mth Subscriber memberships awarded every week for verified new camera reports!

Submit Speed Camera Locations Now


CamerAlert Apps



iOS QR Code






Android QR Code







Terms & Privacy


GPS Shopping