I've heard 18 mpg from my friends. Does sound like a vacume leak. You might have 3:55 gears. Could be a bad vac hose or a leaking intake manifold, etc.
Inertia Spartan + cam, Circle D 3,000 stall ,Sonnax line pressure booster, MagHytec 68RE pan,and differential cover, Vararam, JBA's with cut outs, Rancho 9000's, Helwig rear sway bar,3:92's, extra rear leaf,Line-X Bedliner,GY Armortracs, 245,000 miles To Hell and back. 14.2/96 mph NEDW