I think with a good lockup OD tranny (like the 700r4) and a 3.08 gearset you should at least get your milage into the teens. Another thing to do would be to go to the narrowest treadwidth tires you can find to lower the rolling resistance. That should make a big differance. Also, lower the tailgate or take it out when you drive on the freeway, that helps quite a bit. The tailgate acts just like a spoiler on an airplane, and adds a lot of drag.
