You used to be able to do that in MT. It's been a long time since there was "no speed limit". There never really was that law, anyway, as a cop could give you a speeding ticket for any speed "if they thought you were driving unsafe/too fast). So even under 80 mph you could get a ticket if the cop so desired.
I see out of staters going 90+ (often 95 to 100) on the interstate in MT, and they are usually pulled over 10, 20 or so miles down the road. Always good for LMAO at them. And I'm not that slow driver, I still go slightly above average often, just not enough to get pulled over.
TBF, I did make the 998 mile trip from my place here in MT to my parents place in northeast NE in 12 hours flat one time. That was a long time ago. I was also alone that trip and took the least amount of time possible when I filled up gas. You can do the math and see I was moving pretty fast almost the entire way with two quick fill ups (I hate wasting time at gas stations on this long road trip I have made probably 300 times round trip). I get off the interstate usually at the Reliance exit in South Dakota, heading south to Gregory SD. So a decent portion of the trip is 2 lane rural HWY, but that stretch from Reliance to Gregory I used to really push it. 90+ and it is hilly with a lot of curves.
These days, I set the cruise on 83 on the interstates and that is good enough. Make two stops for fuel, don't waste time at gas stations. Obviously if/when I have kids with me, you gotta take longer stops, but if it is just me, fill up, piss and go.
BTW, the WORST stretch of road to speed on the entire 998 mile trip is from O'neil heading east on 275. I see more cops on that stretch than anywhere. I set it at 68 and let it be. Last thing I want is a ticket after 13 some straight hours of driving.