§ 23.969 Fuel tank expansion space.
Each fuel tank must have an expansion space of not less than
two percent of the tank capacity, unless the tank vent
discharges clear of the airplane (in which case no expansion
space is required). It must be impossible to fill the
expansion space inadvertently with the airplane in the
normal ground attitude.

I don't know whether cherokees have expansion space, but they do have a
fuel vent that vents fuel "clear of the airplane" (as far as I can tell).

However, I would infer from the reg that 2% would be typical of expected
expansion. So, half a gallon loss out of a 24 gallon tank would not be
outrageous.

Am I missing something?

Jose
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