Making fuel???
I remember flying a Chieftain and watching the fuel gage increasing from half full towards the full mark as we flew along. I snagged the fuel gage on the MR, and the engineers could not find any fault.
Next time I flew it the same thing happened and once again the engineers could not find anything wrong with the aeroplane. They were looking strangely at me.
Next time it happened I landed and went straight to the fuel filler cap, and found evidence of a definite suction when I removed it. There was suction distorting the fuel cell gradually as we flew along and lifting the float to increase the reading. The filler cap is in a low pressure area. After the aeroplane had stood for a while, the pressure equalised, and the fault was gone. They told me they fixed it by replacing the seal on the fuel filler cap. (I had already asked them to check the fuel tank vents, and they said "no fault".)
The fuel cell was serviceable.
I prefer solid fuel tanks.