There are several different way to get to Ft Collins CO... all about the same distance as you do not say anything about stops in route. North of Ft Collins I would use 287 to cut the corner from I-25 to I-80. At Rawlings get back on 287 all the way to the Tetons. Sinks Canyon Sp in Lander is worth a stop as is a couple day in the Tetons. From the south entrance to West Yellowstone is a easy route although it can be busy. Eat lunch on balcony overlooking Old Faithful... on the second level of the lodge.