Rob Frankham wrote:
It's a bit dangerous to assume that all BMW speedos are innacurate...
Yes, assumptions are always dangerous and often incorrect.
However, from the factory, as new, and with factory installed tires,
the amount of speedometer error on an airhead is "fixed" by vehicle manufacturer agreement
to be 10% at @ 60 mph.
This calculated amount of error is a conscious, deliberate, and consistent fact.
There will be NO speedometer error at 0 mph, 10% x 0 = 0.
As soon as the mc begins moving – a certain amount of error exists,
and obviously, 10% of 70 mph results in a
greater indicated mph vs actual mph difference
than 10% of 20 mph. A simple ratio calculation.
Airhead speedometers are mechanical / electrical devices, and as such,
will be subject to malfunction due to wear, dirt, moisture, and neglect.
Speedometer error that is greater or lesser than the prescribed 10% can come from several causes
but most commonly is due to nonstandard tire diameter, in which case the error is:
% error = [100x(1 - "new diameter"/"standard diameter")]+[10].
We all want to fixate upon tire width, but even a slightly different diameter / circumference
as measured with the tire inflated and on the rim
than those few tires prescribed in your owner's manual
result in a significant speedometer reading change.
Rob Frankham wrote:The point is, when dealing with speed cameras, that 10% assumed over read can turn out to be quite expensive...
vanzen@rockerboxer.com wrote:
European Union member standards:
. The indicated speed must never be less than the actual speed,
i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
The point is:
If indicated speed is less than actual speed on your airhead,
a situation that would warrant an expensive encounter with an officer of the law or a speed camera,
fault must be attributed to the neglect or ignorance of the owner.
