Remember it very flat round these parts. The highway numbers are very constant on the trip from Houston to Lake Charles 170 miles away. The speed limit varies from 65 to 75, so I keep the cruise control on 70. The mpg averages 24-28 in US gallons unless there is strong wind. We've made this trip many many times.
City driving includes some highway here as we have lots of them
The widest highway in the world is in Houston. Yes I do make a game out of getting the mpg as high as possible. The fuel economy display has greatly changed my driving habits for the better which is not a bad thing.