If, and this is true, automakers have made huge strides in fuel efficiency over the past 30 years, why aren't we all driving the 100-mpg ubercars we were promised at Epcot Center when we were but wee lads and lasses?

The answer is that our cars, like our homes and just about everything else we consume, have been supersized, says MIT economist Christopher Knittel.

Specifically, between 1980 and 2006, the average gas mileage of vehicles sold in the United States increased by slightly more than 15 percent — a relatively modest improvement. But during that time, Knittel has found, the average curb weight of those vehicles increased 26 percent, while their horsepower rose 107 percent. All factors being equal, fuel economy actually increased by 60 percent between 1980 and 2006.

In other words, if cars today were built to the same scale as cars in 1980, when they had an average 23.1 mpg, the fleet-wide average of all the passenger vehicles in the U.S. would be 37 mpg, says Knittel, or nearly the same as our most fuel-efficient conventional sedans.

Instead, the average fuel efficiency from 1980 to the present has been more or less flat. Which is even crazier when you think about how much more we know about making cars than we did in 1980. We could have carbon fiber supercars with aluminum engines, but noooooo, we need Escalades and Tundras so we have something to move into when the bank forecloses on our house because we failed to budget for the long sunset of oil.

But don't take my word for it; here's Knittel on why we're in this pickle. (Hint: it's not the fault of the automakers.)

“I find little fault with the auto manufacturers, because there has been no incentive to put technologies into overall fuel economy,” Knittel says. “Firms are going to give consumers what they want, and if gas prices are low, consumers are going to want big, fast cars.”

If we could return to the average size and power of a 1980 vehicle but continue the trend of increasing efficiency through technological innovation, we would raise the fleet-wide average to 52 MPG by 2020, says Knittel. Ultimate, he believes the solution is up to policymakers, in the form of increased fuel efficiency (CAFE) standards, and a gas tax.