"These butanol isomers, due to their different structures, have somewhat different melting and boiling points. "

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Like gasoline, butanol would be a cocktail of different isomers, each with different flash points. As such the engine cannot be perfectly tuned, and incomplete combustion would occur. Soot would build up, fuel would be wasted, etc. Basically it would be less efficient, that is unless a reduction in vehicle weight resulted in less fuel being used.

I think vehicle weight reduction is the best way to go. Reducing highway speeds would also work but is less desireable, and wouldn't combat rolling resistance. Reducing vehicle weight would kill two birds with one stone, reducing both kinetic energy and rolling resistance. Increasing tire pressure would help too.