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Bringing back a really old thread; this one got me going in the right direction so I'm going to add to it.
I needed to recalibrate my speedo, so I used my GPS to determine the error at 60 mph (it read 65 when I'm really doing 60). I removed the speedo from the case to find the potentiometer. Next I made a simulated speedo sensor with a set of old breaker points, a piece of old distributor shaft, and my drill press. (Drill press spins the distro shaft, trips the points, which provide a signal to the speedo.) I wired up the speedo innards to a bench-top 12V supply and my makeshift signal generator, adjust drill press to a simulated speed of 65mph as read on the speedo, and then dialed the indicated speed down to 60mph.
This may not be as accurate as calibration directly against a GPS, but probably will be "good enough". (I'm doing some rust repair so won't have the car on the road for a while to test.)
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