[Cycle]

So you're just using a different means of measuring the same thing... the volume of fuel (for any given fuel) used is the same mile for mile on average.

BTU/mile still has to take into account the amount of fuel used at some point, because you're not buying your fuel in accordance with its BTU content... you're buying it by volume. You might as well claim propane has a different "cent per mile" efficiency, as well... a difference with no distinction.

And you've still not addressed my main point... that a gaseous fuel exhibits better fuel economy (on a BTU/mile or a MPG basis) than a liquid fuel, even if that gaseous fuel has a lower BTU/volume content.