[RedDevil]

Maybe the rating should be split into 3 categories, reflecting driving style.
Some cars, especially hybrids with an automatic gearbox, compensate somewhat for bad driving style as they regain some of the energy wasted on excessive braking and such.
Other cars, like the tiny turbo ecodiesels, start using loads of fuel when revved.

We need not one but 3 combined ratings; one when driven spiritedly, one when driven averagely and one when driven eco conscious, let's call that the CAH rating.

My car (41 MPG EPA combined rating) may have a CAH rating of 33/41/55 MPG.
My colleagues Skoda Fabia 1.2 TDI would have a CAH rating of 30/45/70 MPG or something like that.
So if you are foot heavy you'd know the Fabia would not save you anything compared to the Insight, while if you hypermile you'd save a lot.

Btw. CAH stands for Clarkson/Average/Hypermiler. Other acronyms are still possible at this stage