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which speed reading is more accurate?

 
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Lorax
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Joined: Apr 17, 2005
Posts: 46
Location: UK

PostPosted: Sun May 29, 2005 2:51 pm    Post subject: which speed reading is more accurate? Reply with quote

Hi,

I noticed that the vehicle speed that tomtom nav3 displays is a good 8mph slower than the reading on my cars speedometer.

Which is correct? Is the gps more accurate or the cars own (i have no idea how the car's speedometer works. I guess it's not a little wheel attached to one of the tires like on a bike Confused ).

Sorry if this has been asked. I couldn't find it in a search.

Lorax
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lbendlin
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PostPosted: Sun May 29, 2005 3:04 pm    Post subject: Reply with quote

Assuming you drive at constant speed - then the speed reading from the GPS is much more accurate than the speedometer. (accurate to 0.1 km/h)

Beware though that the GPS speed gets only measured once a second, so if you change speed within the second (including sudden stops) then the GPS won't be as precise.
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jasb
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PostPosted: Sun May 29, 2005 11:26 pm    Post subject: Reply with quote

Hello


Speedometers are set higher than the actual speed.

jasb
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Lorax
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PostPosted: Mon May 30, 2005 12:14 pm    Post subject: Reply with quote

Quote:
Speedometers are set higher than the actual speed.


Why?
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Robin2
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Joined: Nov 24, 2003
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PostPosted: Mon May 30, 2005 12:19 pm    Post subject: Reply with quote

Because European legislation decrees that manufacurers must supply cars with speedos which read anything between 0 and 10% fast. That is a sensible measure to ensure that, even with manufacturing tolerances, no-one can be driving in excess of the speed limit when their speedo says they are driving at the limit.
My Golf reads about 5% fast and my Saxo about 3% fast.
In my experience, Fords read nearer to 10% fast
Robin
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Lorax
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PostPosted: Mon May 30, 2005 12:37 pm    Post subject: Reply with quote

Interesting. Thanks for that.
So the GPS reading is a more reliable indicator of the speed.

Lorax
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lbendlin
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PostPosted: Mon May 30, 2005 1:35 pm    Post subject: Reply with quote

And just for the sake of completeness - this is only true in Europe. In North Americas the speedos are spot on (but still not as accurate as GPS)
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amcluesent
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Joined: Jul 03, 2004
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PostPosted: Mon May 30, 2005 4:58 pm    Post subject: Reply with quote

>GPS reading is a more reliable indicator of the speed<

As ACPO 'guidance' on prosecution for speeding is posted limit + 10% + 2mph, I've set my TT to have a 'safety speed' of 78mph so I'll know when I'm taking a chance on the motorway.
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bignick
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PostPosted: Mon May 30, 2005 5:14 pm    Post subject: Reply with quote

ACPO guidlines are:

10% + 2mph = consideration of ticketing (fine and points)
20% = consideration of summons (fine, points, loss of license)

Of course, this only applies to if the police stop you. Cameras are different.

Cheers,

Nick
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Robin2
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PostPosted: Mon May 30, 2005 6:42 pm    Post subject: Reply with quote

Yes - where I live people have had penalties for Gatso readings 1 mph above 30 mph
Robin
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novaya_zemla
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Joined: May 19, 2004
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PostPosted: Mon May 30, 2005 9:57 pm    Post subject: Reply with quote

Quite right. Speeding is an 'absolute offence' ACPO guidelines are just that - If the police wish to prosecute at 71mph they can (and in some areas, they do) What this does to the image of the police who are paid out of the public purse to serve the public is quite another matter. Crying or Very sad
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SimonCatlin
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PostPosted: Mon May 30, 2005 10:47 pm    Post subject: Reply with quote

10% + 2 mph over is definately in force on the M4 from Hungerford to Bath as Wiltshire police have said they will prosecute at 79 mph.

However, the Met are know not to prosecute those in 30 mph up to 40 mph (unless using Gatso or Laser) as the paperwork would clog up the civilian staff!
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amcluesent
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PostPosted: Tue May 31, 2005 6:13 pm    Post subject: Reply with quote

>penalties for Gatso readings 1 mph above 30 mph<

Could have been worth some questions under FOI to the CC, asking about the Force's policies re. following ACPO guidlines. If they see hassle rather than an easy £60, might have decided not to proceed...
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