You could drive slower, but that takes more time and might use more gas. With local gas prices now above two dollars per gallon, and continuing power shortages in California, and a controversial new national energy policy soon to be announced, this question is an interesting parable. Local gas prices are up, and so is the average size of the cars I see at my rural school. Conservation means doing more with what we have, not necessarily making do with less. My stumper doesn't depend on what car you drive, though maybe it should.
Gas mileage works the same way.
We get the worst mileage idling at rest, and driving slowly is not much better. Mileage improves with speed up to a point, but air resistance increases with speed squared . Driving twice as fast has four times the drag. At some point aerodynamic drag catches up, and gas mileage declines. The most efficient speed for most cars is between 40 and 60 mph, near the low end of your highest gear.