[trikkonceptz]

Your analysis aside, I think big rigs get better mpg per pound because they have to. Meaning their purpose is to haul in order to make money, passenger cars are simple transportation for individuals.

Therefore the industry put forth much more effort to make those monsters more fuel efficient in an effort to reduce operation costs. Same would hold true for trains. There is less pressure to make improvements in those fields because it serves the greater good, product and services. Whereas commuter cars are expendable and simply a profit center from which to reap benefits.

This also translates to replacement parts. More passenger cars are repaired at shorter intervals than tractor trailers. My guess is for the same reason.