But I would say that most people aren't just looking at one car. They are looking at multiple cars, which car do they want the most? Unless you're super-rich, the cost of the cars will come into play. I'm looking at Cars A, B or C, so which one do I get? When they say 15, 20 and 28 mpg on them I'll get a warped sense of cost between them. I love A, like B, somewhat like C. How much do I save going A-B? Is that worth it to me? How about another step to C? Is that worth it? It looks like an even bigger savings, 8 mpg, so I might get C even though I don't like it as much.
But that's really 6.7 vs. 5 vs. 3.6 gallons/100 miles. Out of my wallet that will be 1.7 gallons or 1.4 gallons different. Going to C doesn't save me as much as I thought when looking at mpg. I'll stick with B.
Forget about the environmental aspect. The less fuel we use, the less dependent we are on foreigners. The less fuel an individual uses, that's more money in the bank. Anything that expresses that more accurately for the buyer is a good thing. I'm not saying everybody should start thinking in terms of gallons/100 miles, but in terms of an informed consumer it would be better to have that on dealer stickers than mpg.
Any "informed" consumer that is getting into as detailed an analysis as you are describing would have a good idea of his annual miles driven. Estimating annual fuel consumption based on annual miles driven is actually easier (one calculation) by dividing annual miles by the miles per gallon figure to determine annual gallons consumed as opposed to dividin annual miles by gallons per hundred miles and then multiplying by 100 (two calculations).
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.