Hills and higher highway speeds make just as much impact as rabbit starts, unfortunately. You can train yourself to drive smoothly and maximize your input to mileage, but it can't fix everything unless you're willing to impede traffic. I am not.
Driving from Houston, through rural Texas, and all then way across Arkansas, I saw my average drop from 35mpg across 100~ miles of Texas freeway at 70mph down to 31mpg at the same speeds on the rural highways, just because of geographical reasons. The long hills of the Texarkana area with long term use of cruise control did not fare well at all.
2013 Focus SE 5M Daily Driver
2003 Mustang GT V8/Auto Wife's Daily
1986 Mustang GT V8/5M Toy
- Jody -