my car has instantaneous MPG on the dash. when i drive at a constant 65mph on a level highway, i get about 27mpg. when i drive about 80mph (like most people here in MI) it goes down to around 23-24mpg.
another factor to look at besides wind resistance is engine RPM. the engine is more efficient at lower RPMs so if you are going 80 and your car has to do about 2800rpm it's less efficient than if you're going 65 and your car only has to run at 2100rpm or so.
And, the best way to get better gas milage is to pretend you're on a bike. when you're cycling, you want to really pedal fast going down hills, so its easier to go up hills because you have all the momentum already. so, when i drive, i push my foot down ever so slightly on down hills, so my speed increases about 10mph, and when i go up hills, i let my foot off a bit, or even take it completely off the gas depending on how steep or long the hill is. this works extremely well. its just important to remember to keep those RPMs down. you never want to push the pedal down more than a little bit. keep everything smooth. my city MPG average has gone up about 6 since i started doing that.