Downtown Missoula at twilight
It depends on the year. Seriously, we've gone back and forth for decades having no highway speed to having 75 miles an hour. The most recent back and forth removed the speed limit to replace it with 75 mph on highways unless going through certain towns. Then it's 65mph. You really need to watch for the signs and pay attention. (You might want to pay attention for deer, skunk, coyote, antelope, elk...)
Then again, some of our highways go through the tiniest of towns. I'm not kidding when I say the highway speed limit is 25mph. If you get caught going over that for the one block or less in the posted area, you will pay a hefty sum to keep the town in finances ;-)
I drove through the town of Belfry, MT. The speed limit was 35 at the edge and 25 through the main stretch. The main stretch wove down little tiny roads the size of alleys past several antique churches and the high school, home of the Bats!
It's just across the border from Cody, Wyoming area heading in the direction of Billings, MT. The country is gorgeous there. The Yellowstone River flows through and dares you to stop and stare. The landscape is dramatically different within a few miles. Anything from forest to desert to farmlands.