Gas gauges on A-bodies

The issue isn't usually the gauge, it's the sending unit. Because of the tire well and how the gas tank on the A-bodies is shaped the level and the amount of fuel left isn't linear. The factory sending units had their resistance set up in a way that wasn't linear to match, but the aftermarket sending units seem to all have a linear resistance set up, with the result being that the gauge doesn't read accurately to the amount of fuel in the tank. And that's assuming that the float and the sending unit arm are set up at the proper angles to begin with, the aftermarket ones frequently require a little tweaking of the float arm to correspond correctly to the actual fuel level in the tank.