When driving over 40 mph, you can increase your gas mileage by:

a. Turning off your air conditioningB. Closing your windowsC. Opening your air vents?

When driving at speed of over 40 mph, one can increase the gas mileage by simply closing the car's windows. This is because at high speeds, open windows tend to reduce the car's aerodynamics which then leads to a significantly lower gas mileage.