Yes, obviously a car with a manual transmission will not downshift on hills.
Generally, cars before model-year 2008 do best at around 55 mph because that was the CAFE test spec, as I vaguely recall. That's how GM got all those "30 mpg highway" ratings... until 2008. Then they bumped it up to, as I recall, 65 mph for most of the time and sometimes higher -- ? we can look it up, of course.
For some cars I'd now expect 65 to be nearly as good as 55 -- but not for a ’97 Avenger automatic. I will have to experiment with our 300C. The Dart 1.4 did insanely well at 60 mph but dropped quickly above 65. The Fiat 500 was similar in my (limited) experience. Our minivan certainly does better as you go slower but it's an old four-speed and with that huge frontal area, you'd expect to be brutally punished for high speed driving.
One reason I get lousy mileage from my '74 Valiant slant six is I don't drive it in its ideal range, which I think was 35-40 mph! Definitely poor mileage as you go up to 70! Amazed I ever got 18 mpg from my old 318.