Roger, When those 15 gallon tanks were first produced and STCd, it was a lot harder to find instruments and radios that fit the Coupe's panel depth. That is greatly relieved these days.
It's no minor matter to sit longer than 18 gallons at 5.4 gph, let alone the extra sitting time you get if you fly high and lean the engine. (6 gallons reserve) And, just where does fuel from the wing tanks gush if they are breached in an accident? All outside or would some or much of it come inside through the wing root? For me, I like Fred's design. Both have advantages and disadvantages. Ed
