Well, after some thought and some research I was able to put it all
together. I used the two speedometers I have available to me which
are both for 3.7 differential cars, and one probably correct report
of a speedo from a 4.11 differential car.
Because the tire options had slightly different diameters, 2 different
speedo calibrations were used for each differential. The presence
of overdrive did not change anything regarding the speedo. The
speedo calibration is based on the number of cable turns per mile
travelled. I figure the cable turns 43 times for every 30 turns of the
driveshaft.
I also figured that 1184 and 1152 were both exact multiples of
32 (37 and 36, respectively). I disassembled a speedometer
and found that the cable drives a worm gear with 32 teeth.
The worm gear pushes a pawl once for each revolution. This
pawl .... taaa daaa..... turns a wheel with 37 teeth! (it is a 1184 speedo)
knowing this I was able to confirm one report of a 1280 speedo
on a 4.11 differential (32*40). This tells me that the other 4.11 speedo
will be labelled 1312 exactl (32*41).
Now, since I have a 4.11 differential, I need a 1280 calibration
speedometer,
even one that is defunct. I should be able to exchange the 40 tooth wheel
with a little difficulty.
BTW, the speedometer calibration is completely independent of the
odometer calibration. The speedometer is set by spinning the
cable input at a known speed and pushing the needle into the proper
position! It is easiest to do this with a variable speed motor turning
at exactly 1152 or 1184 or 1280 or 1315 rpm, and then pushing the needle
to point to 60. Or else you can drive at 60 mph on the speedometer, and
time your actual speed. Then dismantle the speedo, turn the disk inside
the works until the needle points to 60, hold the disk in that position,
then
push the needle to the measured speed. Presto, a calibrated speedo.
-Tony
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