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#1
08-26-2012, 07:29 PM
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Incorrect Speedometer Reading
Anybody else ran into this problem? Should this be an easy fix for the dealership? 
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#2
08-26-2012, 08:48 PM
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I seem to recall Car and Driver running an article on this years ago. It's by design in just about every BMW, and probably all German manufacturers due to German laws. Something about manufacturers being responsible for speedos not reading low even if the customer changes the overall tire diameter. So they purposely make the speedo a bit high just in case.
It's certainly been true for the 3 BMW's and 1 MB that I've owned. They've all been about 2-3MPH high.
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#3
08-26-2012, 08:49 PM
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That is by design, the car actually knows the correct speed reading, and you can code it to display the actual speed.
__________________
 | '23 Tesla MY LR Wife's Daily | '22 Tesla MY P - My daily |'09 E70 X5 35D - retired| '17 F15 X5 35D - Retired | '16 F15 35D - Work Horse | |
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#4
08-26-2012, 11:24 PM
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Welcome to BMW !
My 3rd one, and this one's the closest to the real speed, at +1.5 MPH (3KMH). Each time I change to a new BMW, I use 2 different GPS to "calibrate" the error level. I do 3 readings: (30MPH, 62MPH, 75MPH). Then an average and I know at what speed set my cruise control.
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2010 X5 35d Black - Full (current) 2012 X1 28i (retired, won't be missed) 2007 335i sedan (retired, and missed)
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#5
08-27-2012, 12:07 AM
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In my '06 4.8iS the 20" staggered tires are standard.
The speedometer is reading 4229 impulses (from speed sensor, that might be equal to tire rotations) per one kilometer, while the odometer is registering 4282 impulses per one kilometer. those values are specified in the FSW file but I do not know what would happen if you change those values... It seems that the 4282 impulses are the correct values to represent the speed but, as the folks have mentioned before me, the speedometer is deliberately calibrated to show higher then real speed. the digital speedometer shows the correct speed in km/h...
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#6
08-27-2012, 12:20 AM
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so when I put the bigger staggered 20s on mine and they say my speedo would be off, it's actually right then?
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#7
08-27-2012, 12:29 AM
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this stuff involves some higher levels of math - how many impulses are generated per one full rotation by the speed sensor (hall effect sensor in the wheel hub)... then using the Pie number (what is it? 3.14?) and the diameter of the new wheel, can calculate the new number of the impulses to be programmed into the car...
Not that I even would consider anything like that... hahahaha
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#8
08-27-2012, 01:26 AM
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No math required, just memory. Check and see how close it is, and then drive that much over the limit without worry about getting a ticket.
The BMW spec is 2.4 mph + 10% high, no margin on the low side. Previous discussions here, including a scan of the BMW TSB giving the accuracy range, from 1996. This isn't a new issue. http://www.xoutpost.com/bmw-sav-foru...-possible.html
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