The car and vehicle powertrains are designed to give optimum milage at 55 MPH. After that the milage tapers down. Especially after 65MPH, the drop is drastic.

So if we all drive our cars on or close to 55 MPH, we will reduce fuel consumption by 10% or more on freeway driving.

Reduced speed of traffic is a drawback. But it is more than compensated by better safety at lower speeds.

Here is a blog link.