On my 2400 mile trip, I found that speed had EVERYTHING to do with the MPG you will get. Once you get over the 70+ mph range the MPG drops rapidly. I was getting 25+ MPG when I stayed around 70 MPH (due to bad traffic) but I only got around 19-21 when doing 80+ MPH.
If you are driving 55 (I don't know how anyone can do that - drives me crazy to go that slow) then you will probably see 25+ MPG.
Running 90+ in Wyoming didn't help my overall MPG, LOL.