[RivuxGamma]

Update:

I was cleaning some electrical contacts in case it might be a bad ground, with the positive battery cable disconnected, and I found that someone put the negative battery cable on upside down. So I flipped it over, reconnected it, and, just for the heck of it, tried starting it up. It started fine so I drove it around the block. It shifted just fine through all the gears, the temp gauge didn't act up, the radiator fans weren't constantly running. Still no tach, though. Thrilled, I went about changing the oil, air filter, etc. and parked it. I started it up again later and all the problems that had gone away came back. Also, the horn started working again before I cleaned the contacts or disconnected the battery.

I haven't yet checked the odometer reading with highway markers, but I should be able to tomorrow.