Interstate 80 over the Rockies

Try southern route I10 to I20 to Jackson Mississippi where you catch the Natchez trace (pretty drive) to Nashville, from Nashville you can either go north to Ohio (flatter) or continue east to I81 which is a straight shot although a little mountainous to the Thousand Islands crossing at Landsdowne on the 401. Off 81 is the Skyline drive

How do you think Lewis & Clark felt. LOL

Don't forget the Sierras on the way up to Truckee and Tahoe. Long, long climb from a couple hundred feet ASL to about 6500' ASL, but it's interstate and multi-lane, so twisty, it's not. Once you get over the Sierras, you drop down into flatter country across NV and UT, then have mountains again east of Salt Lake and, as agesilaus said, up to just over 8600' at Sherman Pass in WY.

Lyle

compared to I-70, it's pretty flat ... but windy Wyoming is ... welll ... WINDY! I think I'll take windy (twisty) over windy (blowing) any day.

How could it be flat and at the same time get over the Rockies? The grade is limited to 6% on interstates IIRC. The highest point is the Sherman summit at 8600 ft. If you want flat use I-10