Speedometer wrong?

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MWarren wrote:


Speedo's read high by default. In my '92 535 it's about 5%, measured with GPS. On my old '86 323 it was between 10 and 15%!

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On my '94 540i, I set the speed limit on OBC to 132klm/h and when it sounds, the needle is at 140k!

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SharkmanBMW wrote:

I have the same experience. Had the limit set at 95 mph, didn't come on until 100. Then I lowered it to 90 mph, and then it was 93 instead... Obviously the car "knows" that the speedo is off.

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on my '92 325 my digital speedometer reads 93 kilometers an hour while my analog speedometer reads 100. By inserting a code in my obc a got a little more functions, like a digital speedometer. To bad the 2 don't show the same speed
Erik

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I'll bet the digital is correct. US DOT requires speedometers to be -0 to +5% so most manufacturers intentionally make them about +3% to be safe.
-jim
Erik Metselaar wrote:

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Do you have a source for this?
The only regulation I can find states that "Every bus, truck, and truck-tractor shall be equipped with a speedometer indicating vehicle speed in miles per hour, which shall be operative with reasonable accuracy;"
When did the US DOT decide on -0 to +5%?
Tom
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And 'they' don't define 'reasonable accuracy'?
The first speedometers were chronometric - rather like a clock movement. By the time cars were on the scene, it would be a poor clock that was 10% out - after all, that would be near 2.5 hours per day. ;-)
Later ones were eddy current types - a magnet was rotated within a non magnetic drum. Which pulled it round against a spring, or series of springs, and these could be as accurate as the maker's tolerances and design.
Current types are pulse counting. They might use an ABS disc, or a separate generator. You can buy a pulse counting meter which is accurate to within 1% for pennies these days.
BMW make their speedos over-read deliberately. There is no country in the world that I know of which requires this. The UK requires a speedo to be accurate or 'fast'. -0%, +10% at 30 mph. If it were merely production tolerances then some would be accurate, and some fast up to the limit. But all are fast.
--
*It ain't the size, it's... er... no, it IS ..the size.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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I have a 95 Maxima. I read this conference so that I can get a feel for the BMW machines, I plan to purchase one in the next year. Having read this about the cars reading over, I find it odd.
Occasionally the police drop a Radar Sign on the local roads near my place. I don't know why. I have taken advantage of this facility though and have found that at 50, 60, 80, 100 and 120 Km/h, my speedometer is bang on.
I would have hoped that BMW, charging a premium dollar for their cars would at least be able to make an accurate speedometer.
rtt
London SW

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For what its worth - by checking with radar signs, timing over measured miles, and comparing with the accurate readings from the on board computer, I've found the following BMW speedo errors (all with OEM tires):
1997 Z3: approx. 5% high 1999 328i: accurate within 1% 2003 Z4: approx. 3% high
And as others have said, it is better to be misled by a high reading than a low one, especially when you can easily check it with the OBC.
Of course, the BMW /6 cycle I owned 30 years ago read 122 mph at a true 100, but things have improved since then!
Tom
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It isn't an issue of if they can or not. They can, they just don't. It is common practice that the speedometer will read on the high side because the deep-pockets rulings of recent years might hold the automaker responsible if the car was really going faster than the speedo said, so they make sure the car is going slower. Thank the lawyers, not the engineers.
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Why not just make it accurate? After all, the lawyers will only have your word as to what it was reading. So it makes no difference what it actually reads. If you were quibbling about a needle's width, possibly. But we're not.
--
*Men are from Earth, women are from Earth. Deal with it.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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