The amount of fuel your car burns is related to the power required to move it. There are several components, but, at highway speeds air resistance dominates as it goes as the cube of the velocity. Just looking at air resistance, the power required to move a car at 75 mph is about 3.4 times greater than the same car moving at 50 mph.
There are terms that cover rolling resistance (which tend to have components that depend on velocity to the first and second power), terms for running generators, cooling fans, etc (tend to have a fixed idling component and a term that goes as the square of velocity), etc...
Drive trains are engineered to have "sweet spots" - regions where mileage is highest. On most cars this falls between 40 and 60 mph (there are some cars that get much better mileage at 60 mph than 40, but almost nothing you can buy will get better mileage at 65 mph)
For most people a highway speed of no more than sixty will save a considerable amount of gas. The problem is traffic is often much faster and driving outside of average highway speed can be dangerous ... But a national speed limit of 55 or 60 mph that is enforced would probably make a big difference in fuel consumption for highway driving.
Politicians are too timid to consider measures that might work. They can't even get to the point to make citizens believe there are problems that need to be tackled. grumble - I guess I should be patriotic today and go shopping.