I think you are both thinking more practically than the norm. I know what I want to do and how many miles I want to drive. The MPG figure will help me predict how much that will cost me, and is useful to me for that reason. But, as you have both pointed out, it will is not the best figure to use to make optimal decisions about minimizing my fuel consumption and carbon footprint. The American preference for the MPG figure indicates to me that Americans, in general, are more worried about their own plans, and don’t give a hoot about their carbon footprint.
Needless to say, the bunny huggers at the US National Research Council (whatever that is) don’t share that view.
No, MPG more easily answers "how far can I go on the gas I have?" Gallons/100m more easily answers "how much will this trip cost me?"
But, as you have both pointed out, it will is not the best figure to use to make optimal decisions about minimizing my fuel consumption and carbon footprint.
Both of which are equivalent to "how much will this cost me?"