[TR] Speedo indicator calibration

[Randall]

FWIW, I used a similar process but with home brew electromagnets, to
calibrate a TR3 speedo.  The original head read some 40-50% too high (so 20
mph high @ 40 mph and 40 mph high at 80; so obviously just moving the needle
wasn't going to help).

My first step was to weaken the flying magnet, using a simple "degaussing"
tool consisting of a few dozen turns of wire around a bolt coupled to my
soldering gun handle.  The soldering gun is basically a transformer that
supplies a low AC voltage at high current.  I've used it in the past for
demagnetizing screwdrivers and such, so the connection was obvious.
https://goo.gl/qPnGht

But, even though I tried working in stages, I went too far and weakened the
magnet too much.  So then I had to remagnetize it.  Didn't occur to me to
use a supermagnet (and I didn't have one handy anyway), so I used a simple
DC electromagnet of a few dozen turns of wire around an old U-bolt (from a
muffler clamp).
https://goo.gl/8b71lB

That took it up too strong again, but the second time I was able to hit the
right magnetism with the degaussing tool.

Not to say Tony's method isn't better or easier, just what I did.  I then
used the lathe to spin the speedo at the correct rpm for speeds near 20, 40,
60 and 110 mph and it was spot-on at each speed.  (I made a chart in Excel
from measured lathe rpm in each gear; which you can see in the first photo
above.)

Unfortunately, the needle still wavered after I hooked the trip and odometer
back up; so I'll need to work on that again some day.  Apparently I need to
to a better job of polishing away the rust & pits on the clutch surface of
the number dials.
https://goo.gl/zouL18

-- Randall