So it appears that when selecting either Main tank fuel is used from the corresponding Aux tank first, then the Main tank (this would account for the non linear gauge reaction) but when selecting an Aux tank first fuel is drawn from it and the gauges reads properly, when it is near empty switch to the Main tank. But, I can start on an Aux tank, fly around a hundred miles and watch the gauge steadily decrease, switch to a Main tank and it is full.

I think fuel draw is rather odd, based on watching the gauge selecting either main tank I can fly over a hundred miles and the gauge barely moves, the next hundred miles it plummets to near empty, switch to either Aux tank and they are empty. The fuel gauge is calibrated to 50 gallons at full and monitors whichever tank is selected. Normal procedure is to start with L & R Main, then L & R Aux, then Aux, finally Main. The fuel switch has seven positions - Main, Aux, L Main, R Main, L Aux, R Aux, and Off (Main and Aux correspond to Center 1 and Center 2 in the cfg file).