An old highway cruiser design....
 

https://home.c2i.net/terjekl/carlife.htm

"Just such a car is available now at Oldsmobile dealers. The car represents Detroit's first serious effort at a specific car design to meet the challenge of the 41,000-mile network of interstate super highways to be completed early in the 1970s. Efficient, high-speed, long-distance travel on these highways calls for a very special combination of equipment that cannot be readily "designed" on current option order forms

Oldsmobile engineers have taken the first significant step with their development of the -Turnpike Cruising Package," cataloged as Option L-66. This package is available with F-85 Cutlass Supreme coupes, hardtop coupes and convertibles-models which also are offered with the 4-4~2 high-performance package.

The Turnpike Cruising Package basically is a special 400-cu. in. hightorque economy engine, 3-speed Turbo Hydra-Matic transmission, 2.56 axIe gear ratio, 4-4-2 heavy-duity Suspension, including springs, shock absorbers and front and rear antiroll bars and 7.75-14 white-stripe nylon tires."

"Consider that Oldsmobile engineers believed from the start that the key to the entire problem was use of the lowest possible rear axle gear ratio, while retaining brisk acceleration in the medium speed range. AxIe ratio is by far the most important factor in highspeed fuel economy, much more important than total piston displacement, stroke length, camshaft timing and carburetor calibration. Internal engine friction increases roughly as the square of rpm, and is relatively unaffected by the load on the engine or the degree of throttle opening. The ideal situation for fuel economy then is to cause the engine to lug at low to medium speed. This is accomplished by the low axIe ratio.

Friction loss in a large displacement V-8 engine turning at 3000 rpm actually can consurme more horsepower than it takes to push the car down the highway at 60 mph. lf engine speed can be reduced to 2000 rpm or less, friction is reduced by more than 50%. It doesn't matter that it takes a larger throttle opening to produce the same bhp at lower speed. Reduction in friction loss is far more important. In fact, the larger throttle opening is beneficial in that it reduces manifold vacuum, so pistons have less vacuum to pull against on the induction stroke. The pull against vacuum is known as --"pumping loss" . It also is an important factor in fuel economy, but is not as important as friction loss."

And it had the aerodynamics of a brick.

35-40 mpg would be possible if Cd was cut down to ~.16-.18, with no change in frontal area, weight, tires, or any other parameters. And this is with a large V8.