Be very, very careful about filling up around Disney or Celebration. We stay in Celebration all the time, and some types of rental cars will still be full if we fill up there and then drive to the airport. But others (most noteable Caviliers and Tauruses) will have the needle noticably down when you get back to MCO. See if the needle is still on F when you get to WDW...if not, don't risk paying $5 a gallon to the rental car company.
Barb