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Just a few home truths about C172 fuel systems...
The gauges only need to be accurate at Zero Fuel, so not a really reliable indication in flight.
If using 'Both' the two tanks are inter-connected, so look at the slip-ball to ensure the levels stay the same. If one tank is 4 inches higher than the other, then the fuel levels will ultimately be 4 inches different when landing.
You can even use the slip-ball as a spirit level when parking on uneven grass. If you park with one wheel in a rabbit hole the lowest tank will fill from the higher tank, and could even over-flow whilst you have your tea and scones at the café.
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