re: <E36> OBC/Speedometer discrepancy

["Louis Goldsman" <goldsman@xxxxxxxxxxxxx>]

Peter Biernacki asked -

>Has anyone noticed a discrepancy between the displayed speed on the
speedometer and that displayed on the OBC when you press AVE SPEED and
then the RESET switch.  This should give a pretty instantaneous reading
especially when the cruise control is set to an exact speed.  Any idea if
this is normal or a problem?<

Yes, it's normal and it's probably intentional.  I've had my speedometer
calibrated several times since the car was new.  Every time the readings
were identical (I always had the speedometer checked right after buying
tires).  The actual speed was always less that the indicated speed.  The
error rate was one mile per hour up to 40 miles per hour and two miles per
hour to 75 miles per hour (the highest speed they would test).  The test
operators told me that later model BMWs were all pretty much at this same
error rate.  The OBC reading, when checked as Peter described, always
displays the true speed.

I would guess the error in the speedometer is there to make sure it never
gives a false low reading (what with all the lawyers and police in the U.
S.).  There have also been some comments on the Digest that it may violate
some federal or state regulations to have a speedometer that understates
true speed.

Louis
91 ///M5

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