As others have noted, "driver slower" is such an over-simplification as to be wrong. Anticipation is more important - not so much not using brakes as not needing to use them. Cars have an optimum speed which is usually around 50mph, and which varies from car to car, so driving slower can be worse. (The extreme case is driving at 0mph with the engine running, which will use up all your fuel without getting anywhere.)

AC tends to be more efficient than opening a window at high speed, and less efficient at low speed, with the cross-over point depending on the car.