"Hypermiling" is the practice of saving gas by driving conservatively. Practitioners accelerate slowly, coast wherever possible, and let physics do most of the braking for them. Ardent hypermilers can improve fuel economy by absurd amounts, wringing 62.7 mpg from a 25 mpg car, for example.
This gamification of fuel economy has led fanatics to create DIY aerodynamic modifications. Cardboard, plastic, bondo, duct tape and more are pressed into service: