If you are planning on going through WY I would recomend going 85 to spearfish I90 to gillette and then 75 to Douglas and then I25 to Ft Collins. 75 is easy to deal with and that route takes me just over 11 hours from Bismarck. This is the route I take when we drive down for Broncos games on a regular basis, just drove it 3 weeks ago and there was no road construction and 85 traffic wasn't terrible.