Montana does have speed limits now. Generally 75 but night and secondary roads will have different ones.
Ah.. Thanks for the update. It has been over ten years since I visited.
I would expect that the state had been missing some revenue.
There was quite a bit of back-and-forth arguing for/against a speed limit here in Montana in the mid 1990s. The law at that time only required that drivers maintain a 'reasonable and prudent' speed on the highways during the daytime. Guys who had European sports cars argued that since their cars were made for the autobahn, 110 miles an hour
was safe and reasonable because it the car was designed for that speed. In reality you'd have cars doing 100 mph leapfrogging cars doing 90 mph who were passing cars doing 85 mph...all on winding roads with blind curves, farm machinery, tractor trailers and other slow traffic.
Population centers had posted speed limits of course, so Montana was never completely unregulated. There were also posted speed limits at night and for trucks. Military vehicles were subjected to internal rules to govern their travel speeds. Still, the speed limit for most vehicles was left to the discretion of the driver. For most of Montana I suppose this made sense, especially for the back roads and the highways in the eastern part of the state where it is flat and relatively unpopulated. However, the western part of the state has more dynamic terrain (especially in the mountains) and it has most of the large population centers. I think the law changed because there was a belief that too many accidents were caused by people abusing the lack of speed limits.
I remember arguing in defense of posted speed limits on the highways; oddly, while I wanted the speed limit removed on the interstate where driving is generally safer, the people I talked to who were against a speed limit on the highways actually wanted it for the interstate.
