Most drivers know vaguely that we pay a price for driving faster and faster on the highway: as speed climbs, fuel economy declines -- faster and faster.

But how much faster, exactly? How much does it cost to stomp on that gas pedal? (1/29/13 update: See today's Road Worrier with reader comments.)

Three engineers at the U.S. Department of Energy's Oak Ridge National Laboratory have worked out the answers. They tested a variety of 74 vehicles at 50, 60, 70 and 80mph. For each 10mph increase, they charted the corresponding decline in mpg. ... [MORE]