Wind speed is an other argument. Air resistance is big component at higher speed. And adding wind to that. So you need to some reserve power to overcome those sort of conditions.
On resistance, some cars are purchased to be able to tow a double axle caravan | trailer laden up a hill - on the flat with no towing it's going to be able to go fast.
You could make the argument that with strong headwind you should drive slower to optimize for efficiency.