Gas gauge not working? I think I fixed it.

There are fuel quantity systems that use a variable power source to calibrate the gauge the same way you have done. Search Google for Texas Instruments LM317KCT data sheet. There is a lot of info on it showing how to make a simple and cheap variable power source. Most systems are impossible to calibrate on their full range, so most of them, especially on airplanes, are calibrated to read correctly at empty and not worry about the readings at full (the only time you can have too much fuel, is if you're on fire.)
Until someone makes a plug in system with a microcontroller and a stepper motor indicator, you may only get one point at which the gauge reads correctly.