I'm looking at around 14-15mpg doing half-city and half-highway driving; for a total of 375 miles\tank. If you can swing the expense, I would recommend a Superchip for your Expy. Those of us running them have seen a gas mileage improvement. Couple that with the Gibson exhaust and Airaid intake, results in the improved mileage.
If you go that route, be sure to include your tire size along with your engine code. Prior to shipment, they will need to modify the chip to compensate for the wheels. Be sure to read the Computer Chip forum for all the details...
2000 Wedgewood Blue Expy XLT 5.4L 4WD, C&C group, towing package, beige leather interior, captain's chairs, 6 CD Changer, 255/70R16, all-weather floor mats, factory fog lamps, Mobil1 5W-30 Synthetic w/ Puralator filter
Ford Bug Deflector (2/15/00)
Edelbrock IAS shocks (3/23/00)
Under Seat Locking Storage Drawer (6/6/00)
MPC Rear Air Foil (7/2/00)
Super-White Headlamps and Foglamps (1/15/01)
Gibson Swept Side catback (2/8/01)
Hellwig Rear Anti-Sway Bar (2/8/01)
Ventshade Window Ventvisors (3/4/01)
Stull Billet Grill (4/21/01)
APC Clear Corners (4/21/01)