The following is my thoughts only and not a rule here.
I like to keep things realistic. In my opinion, not only should this thing be fast as heck, but it should also be able to handle pretty well. Something with a trailer or is crazy heavy isn't going to carve any corners. If you can combine handling with a powerful engine and some extra fuel capacity, good on ya.
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That being said, build whatever you would want to run cross country in, and feel could win. Period. That is the spirit of the Cannonball. If a unicycle with a radial engine and a propeller is your idea of a good time by all means build it. Every time we have done a peoples choice poll at the end, the better build has won, not the best designed Cannonballer. If that was the case, I would have won every time.