Drag and MPG

On the highway, more than 50 percent of the energy required to keep rolling is aimed at overcoming aerodynamic drag. As the pedal hits the metal, the aerodynamic drag and rolling resistance increase. That's why the 55-mph speed limit was clamped on the highways in the '70s. Above 55 mph, fuel economy takes a dive. So, by cutting highway cruising speeds to 65 mph instead of 70 mph, gas is conserved. When you use overdrive gearing, the engine speed decreases, reducing both fuel consumption and engine wear. On the open highway, utilizing cruise control maintains a constant speed and can also help reduce fuel consumption.