You can go west all the way on I 70 to Denver, North on I 25 to Ft Collins and N on 287 to Laramie, you can go west from there without big mountains all the way to Utah. There aren't any bad mountains on I 25 from Denver to Sheridan. No grade on any interstate highway is over 9 % or 9 feet for every 100 ft forward. Not sure why no-one could just answer your question. Have run the interstates lots of times and I 80 just west of Cheyenne is a pretty hard pull past the 329 but doable or you can just take 210 and 30, 287 which have relatively flat ground.