The likely thing is it's had an incorrect - probably secondhand -
component fitted at some time. Could be many things depending on model
etc. Even major parts like the final drive.
On another make I came across one model where the speedo read very slow.
This model was available with a wide choice of engines - from diesel to
V-8 through four and six cylinder - all of which used basically the same
gearbox where the speedo drive was situated. But no less than four
different rear axle ratios. The gearbox had been changed by a gearbox
'specialist' and the speedo drive was the wrong one for the rear axle. To
change the drive involved stripping the box as the drive gear is obviously
*Out of my mind. Back in five minutes.
Dave Plowman email@example.com London SW
On Wed, 9 Jan 2008 11:00:01 -0000, "R. Mark Clayton"
We have a winner... This is the most likely cause. Chances are, the
tire circumference is considerably smaller than it should be. This
means that the tires are turning faster to go the same speed which
will run the speedometer on the high side. It's also possiblr that the
instrument cluster has been changed and the wrong coding plug is
installed in the new cluster but that is fairly unlikely. Another
option is that the rear has been changed but this is fairly unlikely
If you have a lease, 15% is a lot. My 45,000 mile, three-year lease
would start costing me $.20 per mile after 38,250 actual miles. So if I
drove 45,000 actual miles I would have to pay $1350 due to the higher
Fit the correct diameter wheels with the original profile tyres. The speedo
is driven from a sensor on top of the diff and the speedo expects pulses
from the hall effect transmitter based on the original rolling
Alternatively, just use GPS/Satnav for a more accurate reading.
Was trying to figure out the typo but it should have been obvious as "-" is
next to "0". While 4~5% is annoying, it seems to be pretty much normal
Greatest error I ever saw was a '76 BMW R75 motorcycle which read 125 mph at
it's top speed of about 103. IIRC, Road & Track magazine in the 60's & 70's
normally published the speedo error at 30 and 60 as part of their road
tests, with some of the worst offenders being the Ferraris - as much as 20%
Wheels are original, tires are correct. The work receipts are
extensive and make no mention of final drive work, which appears to be
I had problems with dash reminder lights and when I went to replace
the dash battery I found it had been replaced already at 60k miles
according to the work records. The rechargeable battery had been
replaced with a lithium battery. I guess the dealer replaces the whole
board assembly at a whopping rate and the replacement is lithium
instead of rechargeable. I just soldered in the same battery as was
there so maybe it's loosing juice already after 4 months. I wonder if
that could give things a false reading. Thanks as always.
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