We are flying into and out of Butte and have reserved a full size car through Budget. We think a minivan might work better for us, but I haven't been able to find a rate with unlimited miles. The best I have found includes 1200 miles for the week and the charge per extra mile is steep ($.25).
I used google maps to map out our point to point mileage from Butte, MT ->Old Faithful Inn -> Colter Bay Cabin ->Jackson, WY-Colter Bay Cabin-> Roosevelt Lodge-> Butte, MT.
This route already has 600 miles. I am not sure another 600 miles is enough