I also have a99 4.0 v-6 auto 5-speed 4x4 offroad. My gas milage is about 12-13/gal, and i had a o2 sensor go or 2,which set off codes for my cat. if you have the 4.0 v-6 then you should have 2 in-line cats and a o2 sensor trailing them. as far as gas mileage, my truck is runnin tip-top with all plugs wires sensors ok. must be the gears in front and rearend.
1999 Ranger xlt 4-door off road splash
3" body lift
2 10,000lb warn front tow hooks
265/75/R16 Wild Cats,stock alloys
Gabriel gold shocks
torsion bar twist 1"
Reese tow pack
K&N box air filter,removed air muff
Alpine deck w/ ipod
4-6" Fozgate 3-way speakers
Fozgate A series 150 amp,bridged
10" Mtx sub
Midland detach face cb radio,Maxrad whip