which speed reading is more accurate?

Lorax

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

lbendlin · Posted: Sun May 29, 2005 3:04 pm Post subject:

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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Lutz

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jasb · Posted: Sun May 29, 2005 11:26 pm Post subject:

Hello

Speedometers are set higher than the actual speed.

jasb

Lorax · Posted: Mon May 30, 2005 12:14 pm Post subject:

Robin2 · Posted: Mon May 30, 2005 12:19 pm Post subject:

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

Lorax · Posted: Mon May 30, 2005 12:37 pm Post subject:

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

Lorax

lbendlin · Posted: Mon May 30, 2005 1:35 pm Post subject:

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 · Posted: Mon May 30, 2005 4:58 pm Post subject:

>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.

bignick · Occasional Visitor Joined: May 23, 2005 Posts: 8

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

Robin2 · Posted: Mon May 30, 2005 6:42 pm Post subject:

Yes - where I live people have had penalties for Gatso readings 1 mph above 30 mph
Robin

novaya_zemla · Occasional Visitor Joined: May 19, 2004 Posts: 23

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

SimonCatlin · Posted: Mon May 30, 2005 10:47 pm Post subject:

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 · Posted: Tue May 31, 2005 6:13 pm Post subject:

>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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