Yes, but it’s not a huge block of the available ponies once you’re at either speed, and there’s probably a shift spot in there in the 50-60 mph range that might be keeping the car at significantly higher RPMs.
Any mph increase will require more hp (fuel), mostly drag and rolling resistance.
At lower rpm in higher gears, you have increased pumping loss.
Caddy had cylinder deactivation with the 8-6-4 to minimize pumping loss back in ~1980, Today it is used on a few models.
NO FREE LUNCH!
And there is no free mpg at higher speeds mostly due to increased drag and rolling resistance.