We're taking the trailer to Fresno, CA in a couple weeks. We were worried about taking I-70 because of all the mountain passes, so we took a test drive to Idaho Springs and back.
It was GREAT! I didn't want to wrap the engine too much so I set the cruise at about 55mph (with the overdrive off, of course) and the trailer pulled just fine! The best part was, going downhill, I guess there was enough air-drag added to the engine-drag that I hardly had to use the brakes (even on Floyd Hill and coming back into Denver on that last, long, steep hill.
So, now, unless there's bad weather, we'll be taking I-70 out into Utah and, hopefully, saving several hours of driving. Plus, the scenery is FAR more interesting!