I don't want to start a "my brand of motorcycle is inherently superior to your brand of motorcycle" argument, but I've heard good things about the newer Harley motors for fuel consumption, and there are lots of people around who know how to customize them. The low-rpm performance and wide-ratio transmissions (low gears for getting a heavier vehicle moving, while still having a reasonable rpm for highway cruising) might make them suitable.
I'm more of a metric-bike guy, but my friends with the HDs speak highly of the build quality and fuel economy.
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