The Wired site writes about the auto X prize competition to be defined and started in 2008. basically it's about a car that gets a 100 MPG (miles per gallon) "efficiency".
That's a strange definition of "efficiency". Seems it's the standard definition in the press. This "Efficiency" is just about "how many mpg", for example: a 100 mpg car would be considered more efficient than a 80 mpg car.
That's not how an engineer would define "efficiency". An engineering (and logical, and correct) definition is: how much resources you use to do a given amount of work. In our example that means: how many mpg for a given car weight. It goes without saying that a 1000 kg. car will do more mpg that a 2000 kg. car. A 2000 kg car that makes 80 mpg is probably more efficient than a 1000kg car that makes 100 mpg. You have to state the weight of the car together with the mpg rating to get an efficiency rating.
So, the Auto X Prize people need to specify the minimum weight for the car, to qualify for the competition.