I suggest we mirror the Europeans and turn our mpg upside down, and start tabulating gallons per mile.  More specifically, gallons per thousand miles.

The problem with MPG is that it is not really very good for comparison.  As MPG goes up, the difference between changes is progressively less important.    For example improve a 20 mpg car by 5 mpg and it saves 10 gallons in 1000 miles.  Improve a 50 mpg car by 5 mpg and it saves 2 gallons in 1000 miles.  Sounds like the same gain, but is a very different improvement.  Flip it upside down though, and it becomes more clear.  Improve a 35 Gallons / 1K Miles (G/1kMi) car by 5 G/ 1kMi and has a similar effect to improving a  50 G/1kMi by 5 G/1kMi.  Both save 5 gallons in 1K Miles.

The Cadillac CTS is rated at 27 mpg highway.  So over a course of 1,000 miles it would require 1,000 miles / 28 mpg = 35.7 gallons of fuel, or 35.7 G/1kMi

The latest Chevrolet Cruze may be rated at up to 44 mpg, or 22.7 G/1kMi.  Clearly by comparison with the CTS, the Cruze will require around 13 G/1kMi less, and I know that gas costs around $2.50 this week, so I can surmise that the Cruze would cost around $32 less to drive 1K miles.  For my annual 15K miles the Cruze would save me 15 x $32 = $480 per year.

On the other hand, a Cadillac Escalade gets around 20 mpg highway, or 50 G/1kMi.  So it would require 24 gallons more than the CTS to go 1,000 miles, at $2.50 around $60 more per 1K.

Why choose 1K miles instead of 100 miles?  So that the variance between models is very clear.  Also it makes it even easier to consider total monthly usage.  If you drive your vehicle 15K miles a year, multiple by 15, etc.

We would still want to measure City G/1kMi and Highway G/1kMi perhaps, and Mixed G/1kMi.

Adoption of G/1kMi as our fuel efficiency standard would help people understand and strive for better fuel economy.