Years ago I saw a web site based (I think) in the midwestern USA, that offered small engines you could mount to a rear shelf on a bicycle. Apparently these things could propel a regular bicycle to about 40mph and still return 100+mpg.
It shouldn't be difficult to propel a regular bicycle at much higher speeds, it's just a question of whether the brakes etc are up to the job of slowing down again from a higher speed, and the implications of coming off at a high speed. I remember years ago I was driving in the Great Smoky Mountains and some of the roads were uphill for many miles at a time. I can't imagine cycling up those would be a whole lot of fun. The Rockies must be even worse.