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Thread: Discuss - Speedo reading

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    Regular Member greeningr's Avatar
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    Default Discuss - Speedo reading

    Noticed yesterday, that the speedo in the Zafira is about 5 mph out (slower) when compared to TomTom's speed reading.

    I do remember that the Demo Vectra was roughly the same.

    What does your's read?
    and would you trust the GPS speed reading over the speedo?

    Discuss

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    Regular Member Spikey©'s Avatar
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    I have an Inforad GPS camera detector, it has no read outs but bleeps if I go over the limit in a camera zone, this is absoloutely spot on with my speedo reading.

    Maybe there's a callibration issue with your speedo, or have you got different size wheels/tyres than were originally supplied with your car? This would make your speedo readings out if you hadn't got it re-callibratred for the new set up.

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    Regular Member Damian's Avatar
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    I have read on various Tomtom forums that this is a regular occurence. The car speedo runs slighty faster than you are going, speedo says 40, you may only be doing about 37.

    I believe from what I have read that if you get a straight road with no bends, no hills and drops and have a good satellite lock the speed reading on Tomtom will be your most accurate speed.

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    Regular Member Andy's Avatar
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    its common for the speedo to read faster than a Sat Nav, eg 100mph on your speedo might read 96mph on the sat nav.

    Be careful though, according to my brother who uses GPS for work on the railways, its currently only accurate to 8metres/second

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    Regular Member C19_CJB's Avatar
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    I use CoPilot software on my GPS PDA and that tells me that I am going slower than what my speedo says by about 2 mph.

    Neither here nor there really is it, afterall you get 10% allowance (I think!?) for the speed cameras

    Chris.

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    Regular Member Ian's Avatar
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    Very difficult for GPS, especially with the very small sensors in these units to detect speed totally accurately. We use GPS at work for surveying in control stations and can take upto 4hrs to get a fix on a single point to +/- 10mm, and thats using a massive 400mm dish. most incar GPS systems are accurate to a couple of metres, so depending on what sytem you have, and any inaccuracy in your speedo, then i would say 5-6mph is deffo acceptable and I would think that the firmware in the car units is conservative - ie shows more than actual, for the likes of speed cameras etc

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    Check the speed on the BC .... its not the same as on the Speedo ...
    If you got standard tyres or the cars cal. for your tires with tech2,
    the BC speed should be true speed.

    In my Astra Coupe i got 96km/h from the BC with 100 on the speedo.

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    Speedos are allowed by law to over-read, but not under-read, so I'd have more faith in the car. At an indicated 30 (on speedo) you'll be travelling between 27 and 30mph.

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    Quote Originally Posted by simonali
    Speedos are allowed by law to over-read, but not under-read, so I'd have more faith in the car. At an indicated 30 (on speedo) you'll be travelling between 27 and 30mph.
    Abs true! .... Its 10% allowed if im remember it correctly.

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    Regular Member Maverick's Avatar
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    Both my Zafira and the Vectra are about 3mph lower on GPS compared to the actual speedo.

    As for my Land Rover, this reads what the hell it likes depending on which way the wind is blowing!!

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