My 96 X-cab, 3.4L, 5sp, 4x4 gets about 20 mpg in the summer and about 18 in the winter. You don't sound far off depending on your driving habits. You can try a tune up but when I replaced the original spark plugs at 130k my mileage went up 0.0! The center electrode was V shaped from the twin electrode but still provided plenty of spark. How old are the O2 sensors? How does it accelerate when you first start it on a cold morning? Another surprise for me was when I replaced the front O2 sensor due to OBD code and sluggish at start up, my mileage still went up 0! Unreal. My previous 93 truck (same config) got 17 no matter what, so I'm happy with 18-20.