Which US state has no speed limit?
Speed limits in the United States
Upon that repeal, there was effectively no speed limit on Montana's interstates for daytime driving (the nighttime limit was set at 65mph) from 1995 to 1999, when the state Supreme Court threw out the law as "unconstitutionally vague."[83] The state legislature enacted a 75mph daytime limit in May 1999.[84]
Speed limits in the United States by jurisdiction
Shortly after the December 1995 repeal of the 65/55 mph National Maximum Speed Law, South Dakota raised its general rural speed limits to 75 mph on freeways and 65 mph on other roads along with 70 on a few 4 lane divided highways. Almost a decade after posting the 75 mph limit, average speeds on South Dakotan rural freeways remain at or below the speed limit. In the urban areas of Sioux Falls and Rapid city, a 65 mph speed limit is posted on Interstates 90 and 29. In March 2015, SD State Legislature has passed the bill to raise the speed limit on Interstate 29 and Interstate 90 to 80 mph. It was signed into law and took effect in April 2015. While the 80 MPH speed limit was initially signed on all rural freeways in the state, the Interstate 90 stretch from Rapid City to the Wyoming Border returned to 75 mph due to safety concerns.