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Thread: GPS v SPEEDO

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    Regular Member savage.'s Avatar
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    Default GPS v SPEEDO

    Have used GPS in several cars and have noticed the speedo is a tuch faster than the speed indicated by the GPS, When the speedo shows 76 or so the GPS shows 70. knowing how GPS works I would say it was the more accurate, but that would mean the speedos I have used are wrong.

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    Quote Originally Posted by savage. View Post
    Have used GPS in several cars and have noticed the speedo is a tuch faster than the speed indicated by the GPS, When the speedo shows 76 or so the GPS shows 70. knowing how GPS works I would say it was the more accurate, but that would mean the speedos I have used are wrong.
    The law allows speedo to 'over read' by up to 10% but NEVER to indicate a
    speed that is less than the vehicle is actually doing. The reasoning is that
    this allows for tyre wear etc - another benefit is that you can not blame the
    speedo when you get NIP'ed - that being the police take on it...

    neilrj

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    Regular Member Ste's Avatar
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    Tear wear would make the actual speed even lower than indicated speed, more so as the tyre wears.

    GPS is always go to be more accurate than the speedo, there are less variables, and the speedo's aren't that accurate even without the additional variables.

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    [Ex]Admin Duncan's Avatar
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    Quote Originally Posted by neilrj View Post
    The law allows speedo to 'over read' by up to 10% but NEVER to indicate a speed that is less than the vehicle is actually doing.
    therefore it's simply easier for the manufacturer to have the speedo over reading a bit - although 6mph @ 70mph is a bit more than usual (it's normally only a few mph over).

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    Regular Member Ste's Avatar
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    Yes, I thought that too.

    What are the tyres like, and are they the correct size for the speedo calibration setting?

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    Regular Member savage.'s Avatar
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    Quote Originally Posted by Ste View Post
    Yes, I thought that too.

    What are the tyres like, and are they the correct size for the speedo calibration setting?
    Tyres are all good on present car only 9 months old with 6000 on the clock. had a GPS in my first vectra (b). Had a set of new boots on it soon after I got it, found the diferance between the speedo and GPS was prity much the same from new to just leagle. Though speedos aren't the easyest thing to read.........needs to be tested on a didgital speedo me thinks.

    as a thought, best MPG is suposed to be achieved at 56mph. but if speedos are not that accurate, how many people have been loosing out while going to all the effort.

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    Regular Member Ste's Avatar
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    Best fuel economy is unlikely to be 56mph, how can every car manufacturer tune their whole engine ranges and applications with different gearboxes, rolling resistances and aerodynamics to give the same speed for optimum fuel economy?

    Truth is, they can't.

    I actually asked Vauxhall customer assistance (who asked the technical folks) for the optimum speed, but they wouldn't give it, nor would they give fuel consumption in all gears across the vehicle speed range at steady state. They said that they didn't have that data (I don't believe them) - my reply was that if they didn't want to give me the data because it is propriatary then fine, just say that is the reason.

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    Regular Member savage.'s Avatar
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    56 is the speed quoted in the 80s (when I got my firs car) by manufacturers when advertising there new cars best MPG for example the metro, cortena, golf. And I have had no reson to question it. If I'm wrong fair enough

    have a look http://www.mpgforspeed.com/
    Last edited by savage.; 20th April 2009 at 14:37.

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