All that said, the 0.5 psi difference at the foot of the electric pump could affect the suction side of the pump enough to change the output pressure.

Think of this way: from 1/3 of a tank and below there is probably 0.5 psi at the input of the pump and with a full tank there is maybe 1.2psi that is over double the pressure at the suction of the pump and is very likey the exact reason explaining the faster when Fuller rather than the 2% difference in the return back pressure.

If you have the old pump it would be very easy to test the theory: run the pump in a five gallon pail of water and spray the output against a wall. If I'm correct the water will spray lower and lower on the wall as the water gets lower in the pail.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)