Don't know if this was your reason for the extra 2.2 gallons in the center tank, but:
If you filled the center tank to the cap, it will probably use about two additional gallons, or eight gallons total. Been there, done that, got melted asphalt to prove it. The two additional gallons will drain into the wing tanks. If the wing tanks are full, the two additional gallons will drain nice and neatly onto the ground. When I first got my coupe, I though that the best way to fill the tanks would be with the center tank. Let the overflow go into the wing tanks and every tank would be full. Well, the nose tank has the drain line set down a couple of inches from the top. I filled the center tank up to the cap. As it drained down, I added more fuel. When the wing tanks got full, I quit adding fuel. The draining continued. The overflow went to the right wing and then onto the asphalt. Now I've got a nice area under my wing, in the asphalt, that will hold all the overflow.
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