What route from Flagstaff, AZ to Great Falls, MT?

89-20-15 is the shortest/fastest on Google Maps

I think the quickest & easiest is I-40 to 93 to 515 to I-15. We take a 40' with toad that way without problem.

Questions about routes in the USA (lower 48) get best responses on Routes. Questions about routes in Canada and Alaska get best responses on the Canada/Alaska board. Sometimes the mods will move a thread. But what it really comes down to is - where are the knowledgeable people hanging out. It's like advertising - where is your target audience.

TexasShadow..Thank you for the info. I will check it out.

Lakeside

You can take 89 north out of Flagstaff up to Provo. The road is good.
I 15 will be no problem for you other than the nuisance of going through Salt Lake City.
If you want to get off the interstate, you can get off onto 20 at Idaho Falls and go up through West Yellowstone and then 287 on north to hook back up with I 15