Well then you had the wrong expectations. If you would have read here (or anywhere), a lifted truck with bigger heavier tires is going to get worse mileage.
I had a 4.0L ranger with 3.73s and 33s, 14mpg.
4.0L sport trac with 4.10s and 33s, 14mpg.
4.0L sport trac with 4.88s and 33s, 14mpg.
You MAY get closer to stock mileage with 4.56 gears. It will be a lot better for performance, but it would take forever to make up for the 2mpg you will gain back.
Again, really, look around at mileage for lifted rangers. There are some factors you may not be figuring either. You need to correct the speedometer, or apply a correction when figuring the mileage as the tire diameter makes the speedometer/odometer inaccurate. Also you are probably running winterized gas. I still wouldn't expect better than 14mpg with that setup. Don't lift a truck and expect good mileage.
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT