@adamrichard

The exact speed at which a vehicle is most efficient is highly variable depending on the car and the conditions but 90kph is definitely much higher than the most efficient speed for most vehicles. If a sports car has a very low drag coefficient, for example, then the most efficient speed to drive at would be a higher than for truck with a high drag coefficient.

Of course there's a limit, you can't continue to slow down and expect increased efficiency. If you slow down to zero and you’re driving a gasoline car the car is still burning gas whether it's moving or not but in most situations, especially on highways if you slow down it’s going to be more efficient.

The air flowing against the vehicle exerts a force that requires energy to overcome and that force increases with the square of the speed. So increasing from 110 to 120kph will cost a lot more than increasing from 80 to 90kph, even though the speed increase is the same.

You can feel this force when you stick your hand out of the window, how much more the force is at higher speeds. You can also see it when you coast. If you coast at a high speed you’ll notice that the car will slow down much more rapidly than it does at a slow speed. That’s mostly due to aerodynamic drag, and that drag costs energy.