In 1974 I drove a 1972 Chevy Monte Carlo from Dallas to Austin which is about 200 miles. This is a drive on I-35 which is pretty much a north-south route.

I drove there bucking a 45 mph wind. I got 4 mpg.

The next day I drove back. Same speeds, same road, same 45 mph wind but now at my back.

MPG was 52.

If I had just rigged up a few sails I think I could have made the trip back for free.