For me the gas pumps always shut off too early, probably due to very poor venting of air as I fill up. I know its not wise to fill up too much since that can negatively affect the system but I do try to listen for the trapped air "burp" noises coming from the filler neck.
Unless I add at least another gallon after the pump automatically shuts off, the fuel gauge always reads a "hair" less than full. BTW I get 27-28 mpg (premium fuel/98'Boxster/5-speed/97,000 miles)
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98' Boxster
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