Exactly, you beat me to it. Fuel efficiency of the engine is just one of many factors.
IN the early nineties, a Euro organisation was trying to do the same thing for various cars and thought for laffs they'd throw in a new flash bus and the then-new Hinckley Triumph. To everyone's surprise, the bike was the best for the environment by a country mile. They assessed the impacts of development, production, sales, usage and disposal. However the caveat here is that the early Hinckley Triumphs were highly modular so the development and production stages had efficiencies not seen by the others.
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