Awesome avatar! I also have a 4.0L and auto. I haven't figured out what it is, but it has tall enough gearing to cruise at 60 under 2k rpm. You may have more of a reputation to uphold, but I was able to actually come out ahead on my aero cap and wheel skirt prototypes. The plywood I scrounged will be worth more than I spent in duct tape.
My uncle did a full exhaust and chip to his, plus a standard canopy and 200lbs storage drawer, and he gets almost 25. So, I don't think that's an unreasonable goal. Suffice it to say he is not familiar with ecomodding. And, at 10 mpg over my current average, I'll be saving a lot!
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