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If the boattail is too "extreme" a project, some sort of kamm back would probably still help a lot. Like everyone else is saying, block the grille, add to the air dam, get the air around the front of the car instead of into it or under it. If there's a roof rack you don't need, remove it when you don't need it, keep the tires aired-up to maximum inflation for mileage, alter driving - hypermiling will probably do as much as all the mechanical/aero tricks.
I drove a 2000 Jimmy 4-door SUV with a 4.3 liter V-6 until it got wrecked, and could exceed 26 mpg with it by hypermiling. Still had the "all around" cargo/passenger space, 4 wheel drive capability, etc. Of course, I had the "White Gnat", too, and for all out gas saving, it would easily double the mpg of the Jimmy, and approach tripling it at times.
A second set of wheels/tires is probably more do-able than a second car, so if you want "meaty" tires for off road, and/or wide tires for your racing, perhaps change tires & run skinny, low rolling resistance tires for economy (assuming you aren't constantly changing roles, like every week, or whatever - that might become a big pain in that case)...
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