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. How we personally decide to use our fuel will make a real difference in future energy policy. I'm way too busy at the end of my Dunn Middle School year to give proper attention to this stumper right now.