Well, fuel efficiency can delay the inevitable a bit, but the bigger problem is that more and more people will be driving cars. (Just take a look at the growth numbers in China alone) More cars mean not only more gas consumption, but it also takes more steel and other materials. (Guess why China is importing a record amount of iron ore and copper this year) We are talking about a finite amount of global resources to meet an indefinitely growing global demand.
That’s why I think the technology cop out (i.e. in the future technology will magically be the answer to everything) is a disservice to the future of mankind. It’s the kind of sand-in-the-head, “so far so good” attitude that’s gonna ensure the end of civilization as we know it sooner rather than later.