how about Billings to Fishing Bridge

westernrvparkowner wrote:
Billings to Laurel to Red Lodge to the Northeast entrance to Tower Junction to Canyon to Fishing Bridge is the most adventurous route. Nice climb and descent over the Beartooth Highway, then another good climb over Dunraven Pass. Might also want to buy your gas in Cooke City, it is a rare opportunity to buy fuel for $1.00 a gallon more than any other place in Montana.
There really isn't any good, direct route available. It is probably best to drop down to Cody, Wyoming and enter from the East Entrance. You could also go to Livingston and drop down to the North Entrance and go to Norris Junction, over to Canyon and down to Fishing Bridge.

What's wrong with going to Bozeman take 191 to West Yellowstone thru the west gate to fishing bridge, pretty simple route