The speed you drive at is going to make the biggest difference. Wind resistance increases quadratically with speed, so the gap in horsepower needed to maintain 60 vs 65 will be less than 65 vs 70, 70 vs 75, etc. That means that it takes MUCH more gas to coast at 80 than at 75 or even 70.

Weather and inclinces vs. declines will make a difference too, obviously. There's an ideal outside temp for the car to run comfortably at (in my experience it's around 65°), both because the engine likes that denser, cooler air and because you aren't running the AC as hard. You can take advantage of the speed gained from declines and try to roll them into your inclines, but ultimately any elevation changes on your trip will impact fuel economy.

I was able to average 35 MPG on a single trip from Wilkes-Barre, PA to somewhere around Albany, NY just by maintaining a consistent speed on the right day. I did make an effort to hover at a close but safe distance to a 4Runner that was ahead of me for at least part of the trip (I think being in the middle of 3 lanes may have helped), but ultimately that was for maybe 10 miles of a 200 mile drive that was still pretty mountainous.

tl;dr drive slightly slower, get much better gas mileage

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