> If MPG was the only factor, I think diesel would be better, since its energy
> density is higher than gasoline's. I know that at my university when they

That is not the only factor towards higher mpg:

1. Gasoline is octane derived (C8H18) whereas Diesel is cetane
derived (C10H22). There is more energy in cetane for not
much more volume than octane (denser - you are correct !).

2. Diesel engines are much higher compression than gasoline engines,
typically 17 to 22 to one versus 7 to 10 to one. Energy efficiency
is a function of the power of the compression ratio.

Typically, gasoline engines are 25% efficient, diesel engines are 35%
efficient (there are many exceptions to this rule of thumb !).

Lynn