Often, the best you can do is to slow down but some setups may have a sweet spot that does best. Keep the tire pressure up and engine tuned. Dont buy the venturi and other mpg scams.
I'm speaking of a Dodge diesel pick up and FW but here are some figures on my speed differences.
65 or so and hills= 9 mpg
65 or so and more flat slight rolling hills = 10 mpg
60 flat/ slight rolling = 12 mpg
55 flat/ slight rolling = 13.7
These are expressway figures on a 200 mile run.
These figures are from the dash display, not paper and pencil.