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Speedometer Calibration
Posted by Jeffrey Sutherland (more from Jeffrey Sutherland) on Thu, 10 May 2001 21:42:44
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

I need a little help with some mechanical theory here:  In trying to
figure out the speedo error generated by my summer tires on my '88 9000
turbo I first measured the dynamic circumference of my winter tires
(195-65 Hakka NRW's) which seem to put the speedo right on the mark.
The OE tire size (205-55) yields about 871 revs/mile as opposed to 832
revs/mile for the Hakkas, which is about a +4.5% error.  Again, I
measured this by rolling the car forward exactly one revolution then
measuring the distance the car moved.  The speedo is off somewhat
farther than that, closer to 10% with these tires.  Any idea what might
be going on here?  In looking at the manual, the 5th gear ratio is
listed as 2.83:1.  The book says that this is the 'total' gear ratio.
Hmmm...  Multiplying 832x2.83= about 2355 which seems to make sense as
this is pretty close to the tach reading at an indicated 60mph.  So, I
guess I've got the right tranny in this thing (5th on a 9000S is 3.13:1
which would be over 2500rpm at 60mph).  As far as the calibration goes,
I suppose I just need to make a placard from some data collected with
the GPS receiver :
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