Can '64 Dart Temp Gauge Be Adjusted?

That factory tool would just be basically a rheostat that you could use to dial in a certain resistance to check gauge. You could do same somewhat, by removing wire at sender and grounding it, with key on. Gauge should go all the way high or low. Then maybe use a high impedance (old)l test light, or an old parking light bulb and socket assembly, and run it in series with sender wire to ground, and look for change in gauge reading. It obviously would help if you knew resistance range of gauge, and then measure resistance of checking device for accuracy. You may just have a bad sending unit or engine temp isn't getting as high as you think, or high resistance in circuit ( bulkhead connector).