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Speedometers

tin79 · 2008-10-17 12:33:42

Hello Everyone First post on this forum, but i have been lurking for a while :D and find the site full of good information and entertainment. Something I was wondering about ... while driving on the M50 i have been watching the speed clocked on their speedometer boards and comparing it to the display on my car speedo (only…

17-10-2008 12:33pm

#1

tin79

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Hello Everyone

First post on this forum, but i have been lurking for a while and find the site full of good information and entertainment.

Something I was wondering about ... while driving on the M50 i have been watching the speed clocked on their speedometer boards and comparing it to the display on my car speedo (only when i am a good distance from other cars so the boards dont pick up the wrong car), anyway i have been keeping a close eye on my speedo and comparing it to the board.

When i have been doing 65, or 70 km/h the boards have been reading 60, 63km/h. The boards always seem over by around 10% and so I was wondering is it just the speedo in the car being set to read higher then actual speed (which i assume it is) or are the boards over estimating in order to get people to stay slow.

Do manufacturers set the speedo over on purpose or is it just loss of callibration over time?

Incidentally when i was just approaching a board the other night someone flashed past me in the outside lane. The board read 150km/h

Thanks for any info

Cheers

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Comments

#2 17-10-2008 12:36pm

Kimbot

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The speedo is calibrated slightly higher so that the company can't get blamed for you speeding if it was set lower!!

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#3 17-10-2008 12:41pm

GigaByte

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Could be either one.

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#4 17-10-2008 12:47pm

unkel

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tin79 wrote: »

First post on this forum

Welcome to boards.ie

tin79 wrote: »

Do manufacturers set the speedo over on purpose or is it just loss of callibration over time?

We've covered that regularly here. See this thread. Summary:

Any speedometer can legally only indicate anything between the real speed and the real speed + 10% of the real speed + 4km/h. Example: a real speed of 100km/h must be anything between 100km/h and 114km/h indicated on the speedo

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#5 17-10-2008 12:53pm

-Chris-

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I passed 2 boards on the M50 yesterday that, compared to both my SatNav and speedo, were out by 5-10KMph.

I think those signs are meant more as a deterrent (in the form of "look how easy it is for us to know if you're speeding") rather than as an accurate representation of your speed.
I don't see how something as rudimentary as those signs could be expected to be accurate.

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#6 17-10-2008 1:44pm

tin79

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Hi

Thanks for the replies, and the link Unkel.

i wonder what the speed detection method on the boards is, is it laser based or more like Gatso technology?

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#7 17-10-2008 1:48pm

-Chris-

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I always presumed it was radar, but I know nothing about these kind of things.

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#8 17-10-2008 2:27pm

gofaster_s13

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Radar detectors pick them up on radar bands

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