... Morris explained the speedo is only a rev counter and anything that changes the number of revs per mile necessitates re-calibrating.
If one left the speedo internal gears the same when switching from a 4.11 to a 3.54, the indicated speed would be 86% of that previously shown given the same size tires.
Yep. The internals of the tachs and speedos are almost the same; the only difference is the speedos have geared ratchet mechanisms to drive the odometers and trip meter. The only adjustment that I'm aware of is placement of the indicator. There is one component in the speedos--tachs may have it too, it's been a while since I've gotten into one--that I don't understand: the only thing that holds the tiny shaft that turns the indicator is a parallel, brass bracket (crappy ASCII art follows):
___X_|_____
\__X_|____/
(the misaligned vertical bars are supposed to represent the indicator drive 'shaft'). The Xes represent a tiny screw that appears to be used to adjust the two parallel parts of the bracket (represented by underscores in my crappy artwork). The only thing I can figure is the screw is used to 'pinch' the two parts of the bracket, which should change the angle of the indicator drive cup to the rotating magnet, which might compensate for variances in different speed ranges (even the 'famous' documentation of the Smiths speedos doesn't mention it).
"If one left the speedo internal gears the same when switching from a 4.11 to a 3.54, the indicated speed would be 86% of that previously shown given the same size tires."
Yep; except I went from the stock 3.91 in a BJ8 to a 3.54, which I calculated was an 11% difference. When Klaus changed the internal gears for me he neglected to recalibrate the indicator, and I passed a California Highway Patrolman because I thought he was doing 50 in a 55MPH zone, and he promptly pulled me over. I was so ******-off at the speedo I handed the CHiP my American Express card instead of my driver's license; and when I explained the problem he laughed and sent us on our way.
Note your speedo--carefully calibrated to your exact tire diameter--will become increasing incorrect as your tires wear down, or you install new ones with a different diameter. First thing I do when I take my long road trip every year is to 'calibrate' my speedo against my co-pilot's GPS.