Montana Traffic Laws
Speed Limits:
Urban district: 25 (ref. 61-8-303, MCA)
Construction zones: 35 (ref. 61-8-303, MCA)
State highways: Reasonable and prudent (day) , 55 (night) (ref. 61-8-303, MCA)
Interstates: Reasonable and prudent (day) , 65 (night) (ref. 61-8-303, MCA)
Speed limits are absolute for all roads, meaning you may not exceed them under any circumstances.

So... maybe Montana would be a good vacation destination.
I also think it would be much more "reasonable and prudent" to drive 140 mph in my Challenger than my Neon...