Here is the section of the article that was referenced. Probably easier to read in pdf form, but posted here if you don't want to deal with the file.
"SPEEDOMETER: Calibration
One very observant person pointed out to me that there is a small dot at about -5 mph
on the dial (on tachometers too). He found that when he pulled out the stop post on which
the pointer rests when reading “zero”, the pointer came to balance pointing at the dot. MG
speedometers have a line in about the same position. If you are restoring a speedometer
without appreciably changing its calibration, then you can probably use that dot to set the
speedometer. Unless the hair spring (return spring) or the magnet have lost strength over
the years, then that dot ought to provide a fairly accurate initial calibration. If you are
recalibrating the speedometer, then that dot will not be accurate at all and you will have to
use the calibration technique outlined above.
One note, if you are recalibrating the speedometer to a setting considerably different
from the original calibration, there will probably be some residual error particularly at the
low speeds. At zero the pointer will have significantly more or less tension against the stop
post. This may result in considerably more or less tendency for the pointer to move at low
speeds resulting in low speed error. You can see that this is already an issue for the
speedometer by looking at the noticeably smaller space between 0 and 10 than between
10 and 20. This is due to the rest pressure of the hair spring on the pointer. At zero mph,
there is NOT zero force on the pointer. This requires the magnet to have to turn at some
speed just to get the pointer off the rest post. (about 5mph). Due to the error you might
experience, it is best to calibrate the pointer to the speed you are most concerned about,
such as the national speed limit. When calibrated to give no error at a certain speed, that
is the ONE speed you can be positive about. Speeds close to that will have minimal error
as well.
If you find that there is a significant error at higher and lower speeds, then the magnet
wheel and hair spring are incorrectly matched. Refer to Appendix IV for one reader’s
approach to this problem.
Driving Calibration
Calibration of your speedometer is easy. If you have not serviced it, it ought to be
close to the correct speed unless you have installed tires of a significantly different rolling
diameter. To fine-tune your calibration, you need to drive a measured mile at exactly 60
mph indicated on the dial (you may use any other speed as well, but 60 is a nice mid-range
number to use). Use a stopwatch to time your mile trip. Your actual speed is 3600 divided
by the number of seconds it took to drive the mile. Now remove the speedometer from the
dash, and remove the works from the case. Do not remove the pointer or face. Move the
pointer to the indicated speed you used in the mile trip (60mph). This is done by gently
holding the drag cup in the works. Double check that the pointer is still at the indicated
speed. Now gently push the pointer to the calculated speed while being sure that the disk
does not move. Now pull up some slack speedometer cable into the dash and reconnect
the works of the speedometer without reinstalling the case. Carefully perch the speedometer
in the dash opening without allowing the dash to contact any moving parts. Time
another measured mile at exactly 60. If the time is not very close to 60 seconds readjust
the speedometer. You should be getting very close to perfectly calibrated with one or two
resettings of the pointer.
Bench Calibration
If your speedometer has been serviced, you can do a bench calibration quite easily.
You need the speedometer, a tachometer, a speedometer or tachometer cable, and a drill
that will turn about 1000 to 2000 rpm. First you need to find out exactly how fast your drill
turns. Attach the drill to the drive end of the cable and attach the tachometer to the other
end. Turn on the drill to maximum RPM in REVERSE. Watch the pointer of the tachometer.
It should be quite steady. If it wavers significantly, either the drill does not turn a
constant speed or the cable is binding. You can lube the cable by removing the center wire
and greasing it lightly with lithium grease. Once you achieve a fairly constant reading, you
can determine your drill RPM by dividing the reading by two (the ratio is indicated on the
tachometer dial face: “2-1”). For instance my drill showed exactly 2400 rpm, so it was
turning exactly 1200. This happens to be the maximum speed listed on its label.
Now you need to calculate what the speedometer OUGHT to be reading at that RPM.
You need to know one of two things. One is the calibration number printer on the dial face
of the speedometer just above the “MPH”. This is the cable RPM that it takes to indicate
60 MPH. If you know that your tires are substantially different from those originally supplied
with the car, you can not use that number as your final setting. If you DO know the
engine RPM that corresponds with 60 MPH, then divide that by 2.5 (known correct for the
TR2-6 series, other transmissions may differ. I suspect that Spitfires used 3.5) to determine
the cable RPM that corresponds to 60 MPH. If you are not sure of this data, then set
the speedometer to the printed calibration and then do the driving calibration discussed
above. As an example, you want to set your speedometer to 1152 calibration. You need to
calculate what MPH should be indicated when you run the drill at its maximum speed. The
MPH will be 60 * RPM / 1152. So, in this case the MPH = 60 * 1200 / 1152 = 62.5. Now
turn on the drill and look at the indicated speed on the speedometer. Turn off the drill and
hold the disk with the pointer at the observed speed. Gently push the pointer to the calculated
speed (62.5) and retest. Adjust the pointer until it reads exactly the calculated speed.
Now you can attach the speedometer to the cable in the car and drive a measured mile
and adjust as outlined above. These adjustments will set the speedometer only. It will not
set the odometer, which is gear driven as stated below."
