Put about 120 miles on the Ranger today, 80% at 60, 65 MPH.
Average speed says 36 mph and mileage is still at 24.6 - 24.7
I'm thinking neither of those are going to change unless I start pulling a trailer.
Just about 9000 miles on it so far, since the end of April when I picked it up.
Suspect you will find a sweet spot for mileage in that 60-65mph range.
For reference my F150 recent 200 mile trip mostly 65mph (~15% at 55) got about 24 mpg. But last year flat highway in OH at 70 it got about 20 mpg.
My '97 Ranger has never got over 20 mpg.