The best highway speed for gas mileage is a balance between engine efficiency and wind efficiency.

The best speed for wind efficiency is zero. If the vehicle is not moving, it's doing no work to move the air aside.

However, the engine is consuming fuel a zero speed. So, this is hardly the best speed for the highest efficiency.

Somewhere around 40 mph is a point where the wind resistance is fairly low and the engine is not doing too much work to move the air aside. So, in reality, it is consuming just slightly more fuel than it otherwise would at zero speed.

However, the rpm's of the engine are higher at 40 mph, so additional fuel is required to achieve the higher rpm's.

Above 40 mph, the engine consumes significantly more fuel for each 10 mph increase in speed, and, additionally, must overcome the higher friction of more rpm's.

It's possible to achieve the most efficient speed at 45 mph if the vehicle is geared tall enough and the rpm's are low enough. The additional work to push the air aside is offset by the lower engine speed.

Anything above 45 mph will result in lower fuel economy.