The National Maximum Speed Limit (NMSL) was a provision of the federal government of the United States 1974 Emergency Highway Energy Conservation Act that effectively prohibited speed limits higher than 55 miles per hour (89 km/h). It was drafted in response to oil price spikes and supply disruptions during the 1973 oil crisis and remained the law until 1995.

While federal officials hoped gasoline consumption would fall by 2.2%, the actual savings were estimated at between 0.5% and 1%.

The law was widely disregarded by motorists nationwide, and some states opposed the law, but many jurisdictions discovered it to be a major source of revenue. Actions ranged from proposing deals for an exemption to de-emphasizing speed limit enforcement. The NMSL was modified in 1987 and 1988 to allow up to 65 mph (105 km/h) limits on certain limited-access rural roads. Congress repealed the NMSL in 1995, fully returning speed limit-setting authority to the individual states.

The law's safety benefit is disputed as research found conflicting results.

The power to set speed limits historically belonged to the states. Prior to the NMSL, the sole exception to this occurred during World War II, when the U.S. Office of Defense Transportation established a national maximum "Victory Speed Limit" of 35 miles per hour (56 km/h), in addition to gasoline and tire rationing, to help conserve fuel and rubber for the American war effort. Although it was widely disregarded by many motorists, the Victory Speed Limit lasted from May 1942 to August 14, 1945, when the war ended. Immediately before the NMSL became effective, speed limits were as high as 75 mph (121 km/h). (Kansas had lowered its turnpike speed limit from 80 mph (130 km/h) before 1974.) Montana and Nevada generally posted no speed limits on highways, limiting drivers to only whatever was safe for conditions.