All of a sudden the temperature gauge of my 63 Dart reads too low with the needle usually in the too-cold zone about 1/16" to the left of the right boundary mark. Even after stop and go driving for 1/2 hour in 90 degree weather the needle got only about 1/16" past that mark into the OK temperature zone. Five minutes later I stuck a meat thermometer into the radiator and read 160 degrees. Since I have a 165 degree thermostat I presume that my cooling system is OK and that therefore either the temperature sending unit or the temperature gauge is the problem.

I'll probably just swap in a brand new sending unit which Kragen's sells for all of $8, but I thought I'd run it by you guys first. What say you? Clearly I've got to fix this else one day I'll be cruising down the highway fat, dumb and happy with the coolant at 210 and the needle at 140.

Your thermostat is too cold. Put in a 180° unit, preferably a Stant Superstat # 45358. There are several very good reasons not to run a thermostat colder than 180°, and no good reasons to run the colder units.

As for your gauge issue: the temp gauge sender could be skewed — careful, the '63 sender is not the same as the '64-up sender; you may want to shop at a better-quality parts supplier than Kragen — but your instrument cluster voltage limiter could also be faulty. Has your fuel gauge's behavior changed any?

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Too many people who were born on third base actually believe they've hit a triple.

If your gauge still doesn't read right after following Dan's advice, but the fuel gauge is OK, it could be due to a corroded connection at the bulkhead, bad connection, or failing gauge.

If you temporarially replace the sender with a 10ohm power resistor to ground, it should read HIGH; while a ~77ohm resistor should read LOW. The gauges aren't very precise, but it should be close. Good luck.