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  1. #1
    Roy
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    Speedometer...

    Why is the reading on the dashboard speedometer 3 miles faster than the actual speed read on my gps? Is the gps accurate? Does this bother anyone that the dashboard speed is not accurate?

    -Roy

  2. #2
    FloydR
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    Re: Speedometer...

    We all care, but that's the way it is. The reason is tied up in some conflicts between European and US regulations: all european cars' speedos read low by some increment so that they meet those regs without having to have different guages.

    The GPS is probably accurate to less than 1 mph.

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    Trust your GPS for sure.

    I've owned lots of cars and there's not a one that isn't just like yours. They're all optimistic from the beginning of time so you can't get in trouble with the law and blame them. And if you'll check more systematically, you'll find your error is actual closer to a % than a fixed error thruout the speed range. I calibrated my own speedo at 100 and sixty to learn this.
    No, I guess it doesn't bother me as it's never going to be any different.

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    GPS relies on DHOP and DVOP calculated figures

    The revolution counter on the car works going up hill, down hill, around corners, under a tunnel, even down the main street of cities like New York. The GPS however relies on the geometeries at an instant in time (because the Satellites are actually moving as well) to get an accurate "estimate" of your location. As for the speed and distance travelled accuracy "estimates" these are variable as well due to the DHOP and VDOP (Horizontial and Verticle Dilution of Precision), it's all spacial geometery mathematics.

    GPS is pretty accurate now, but I think the car odometer will never be based on GPS technology.

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    I almost had my arms completely around

    your explanation, but I totally agree with your negative view of using GPS for an odo. Current one has stood the test of time and is cheap. How about when GPS isn't getting a reading for a while during bad weather or whatever? Or you could put tinfoil over it while you run errands?

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    I hope you were wearing the Hemmings Glove


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    Not sure that one exactly translates to here?


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    most all cars read "off" to some degree. Your

    GPS is accurate. Some car speedos read high...some low. I think most manufacturers say that +/- 10% is within spec. Different sized wheels and tire aspect ratios will affect actual speed. Most BMWs (and other brand cars too) have at least 2 different sized wheels from which to choose (OEM). Even though aspect ratios are respective to the size of the wheel there will still be a slight difference in overall wheel/tire diameter. There's one difference in actual MPH right there.


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    Cars speedo should be calibrated to the

    tire size supplied on that particular car. It used to be done by changing the size of the gears in the speedo. Now I'm sure it is an electronic setting.

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    how about all of them GPS controlled...it

    shouldn't cost that much to have an accurate reading. And how does this figure into the odometer and warranty mileage? Are we getting hosed on our mileage? A couple K miles could prove very important.

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    Just cause the speedo is optimistic, doesn't

    necessarily mean the odo is wrong too.

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    Yep...but it would be nice to know if it is...

    or not. I have a DC trip theis weekend and if I can figure out how to decipher miles driven on my NUVI I'd like to compare it to my odo.

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  13. #13
    Tom K.
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    Is this your first car with a speedo error?

    Speedometers have been off for years - my '76 BMW motorcycle was over 20% optimistic at high speeds, as were some '60s Ferraris. It was so bad that Road & Track magazine used to publish speedomter accuracy at 30 and 60 mph as a normal part of their road tests.

    Most European speedometers are set to read any where from 1% to 10% above the true speed as a social convention - sort of like the top speed limiters. The idea is that it is safer to err by going slower than you think than faster. If you don't have a GPS, my experience with BMW cars is that the OBC reading is also pretty accurate.

    Also, the odometer reading is separate from the speedo, so the mileage (and warranty perios) won't be affected.
    Tom

  14. #14
    Roy
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    Re: Is this your first car with a speedo error?

    My 2002 330ci coupe was only 1 mile off than my gps.

    -Roy

  15. #15
    Tom K.
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    Re: Is this your first car with a speedo error?

    I think you were lucky with the E46 - although BMW seems to be getting better in this regard. My '08 128i also has a very accurate speedometer - although the odometer actually underreads by about 1 1/2%. That would extend my warranty by 750 miles, but my 48 months will probably run out first.

    The trick with the speedometer is to ascertain the amount of the error and then factor it in...
    Tom

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    Change the aspect ratio of your tyres if worried

    The National Australian Design Rules (ADRs) now state that all cars must have speedometers that display 0% to +10% of the actual road speed, it use to be positive and negative tolerances. The European car manufactures were ahead of the game.
    The ADR is to help the State Governments convict speeding drivers - the speedo error error defence is no longer valid with new cars.

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