Interstate 80 over the Rockies

As noted, except for the Sierras, the Wasatch just east of SLC, and one pass between Cheyenne & Laramie in Wyoming, it's pretty flat. In fact, once you get to Cheyenne, you'll be going very, very gradually downhill. Lots of trucks - I-80 is the workhorse of America - and Wyoming is quite windy (springtime the worst, I suspect). For that matter, it can be windy across the plains anywhere; heading east, you'll catch either a crosswind or a tailwind, and the latter can be quite helpful to the mpg.