Interesting thought, Colonel, but mandating an increase in fuel mileage for motor vehicles would result in only the slightest decrease in our dependency at best (and then only temporarily), and very possibly an increase. Greater fuel mileage would mean more people on the road, more people driving, guaranteed. Besides, have you seen the number of vehicles on the road with monsterous V-8's? Americans live and breathe horsepower and torque and if fuel mileage gets altered so do the ponies. I highly doubt even you would want to be caught dead in a Yugo looking creature attached to a chainsaw engine. Or, to be more realistic, a much lighter weight vehicle than you currently own with cheaper safety equipment, because the easiest way to increase fuel mileage is to lighten the vehicle weight. There comes a point when it becomes ridiculous, uncomfortable and dangerous. Remember the first Japanese cars? Little boxes, made for short people, seats uncomfortable, made of two or three beer cans.
Now, if you try to alter the powerplant, you run into more problems. As I stated earlier, people like horsies. Women like large, safe vehicles. You can't operate a Yukon on a Toyota four cylinder, or a six for that matter. You would severely limit the lifespan of the engine if you were to do that. And get laughed at by the driver of the '66 Beetle that blew your doors off at the light. Currently, nobody makes a production vehicle that isn't controlled by a computer, sensing a minimum of 4 parameters and adjusting ignition timing, fuel pressure, vacuum control, and intelligence quotient several times a second. By changing fuel economy limits, more emphasis will be placed on electronic controls, in turn increasing the basic cost of the vehicle, increasing the maintenance and repair costs.
No, CAFE isn't the way to go. There would be a larger impact on consumption if folks remembered what the speed limit was, kids didn't start driving until age 18, people kept their vehicles properly maintained (it would help greatly if the manufacturers made it easier for shadetrees to work on their own cars), and localities figured out how to time stop lights correctly to allow for a smoother traffic flow. But, that's just my thoughts.