I-70 from Utah to Denver.

In July of 2012 we did the I-70 route from north of Moab all the way to St. Louis. The climb up from Vail, and then the next one up to Eisenhower was slow, but hell, if our creaky old 88 Bounder can do it at the sparkling speed of 28 mph, then anyone can (weather permitting, of course...) The downslope past Idaho Springs and on into Denver was no biggie at all, even in the rain we hit.

Once past Denver, though, it's deadly boring for the next, oh, say, 800 miles or so... :)