Today I decided to take an incremental step towards replacing the temp
gauge in my 83 (the light is misbehaving because the capacitor that
controls is on the, er, blink).

I notice on both the temp and the gas gauges that there are two holes
in back of the gauge that reveal a tiny adjuster gear like the one on
rear brakes. I tried adjusting the gas gauge (it read way too low,
that's why I replaced this instrument cluster years ago) so I tried
bringing it up with a small screwdriver and bingo!

There is an upper and a low limit adjuster, presumably, on both, which
means that if you know what value the upper and lower settings should
be, then you could apply that voltage and get the instrument right on
the money.

Unfortunately, you can only get to the adjusters from the back, so
it's not good for seat of the pants setting. But it's there.

Jim