The 1991 Daytona 500 By STP was the first stock car race of the 1991 NASCAR Winston Cup Series season and the 33rd iteration of the event. The race was held on Sunday, February 17, 1991, before an audience of 145,000 in Daytona Beach, Florida at Daytona International Speedway, a 2.5 miles (4.0 km) permanent triangular-shaped superspeedway. The race took the scheduled 200 laps to complete. Following a period of three late-race cautions, Morgan–McClure Motorsports driver Ernie Irvan took the lead from Dale Earnhardt with six laps left in the race. With three laps to go, Earnhardt spun and collected Davey Allison and Kyle Petty, bringing out the final caution with two laps left. Coasting to the finish on fumes, Irvan was able to take his second career NASCAR Winston Cup Series victory, his first victory of the season, and his only Daytona 500 victory.^[1][2][3] To fill out the top three, Junior Johnson & Associates driver Sterling Marlin and RahMoc Enterprises driver Joe Ruttman finished second and third, respectively.

Daytona International Speedway is one of three superspeedways to hold NASCAR races, the other two being Indianapolis Motor Speedway and Talladega Superspeedway. The standard track at Daytona International Speedway is a four-turn superspeedway that is 2.5 miles (4.0 km) long. The track's turns are banked at 31 degrees, while the front stretch, the location of the finish line, is banked at 18 degrees.

This race began a series of changes to pit road procedure after the death of a Melling Racing rear tire changer in a pit road accident at Atlanta the previous November.

• Changing tires under caution was banned, under any circumstances. Any tire changed under caution (even if it was flat) resulted in a one-lap penalty.
• The signboard man was banned from standing in the pit lane. Instead, teams would utilize signboards on a long pole (i.e. the "lollipop") held by a crew member behind the wall.
• Each car was given a sticker based on their starting position. The sticker was placed on the steering wheel for easy reference. The pit stall selection was staggered to prevent drivers in neighboring stalls from pitting at the same time:
  • Cars starting in odd-numbered positions were given a blue sticker with a white 1 on it; their pit stalls were selected from the odd-numbered stalls.
  • Cars starting in even-numbered positions were given an orange sticker with a white 2 on it; their pit stalls were selected from the even-numbered stalls.
• After a restart from a caution, the pits would be closed. On the second green lap, a blue flag was waved at the entrance of pit road, allowing only the cars with the blue stickers (odd) to pit for tires. On the next time around, an orange flag would be waved, allowing the cars with orange stickers (even) to pit for tires.
• A new pit road speed limit was implemented at all times. During cautions, a second pace car was added, which would lead the cars entering the pits, and set the speed down the pit lane.
• Penalties were severe: a 1-lap penalty for changing tires during a caution or pitting with the wrong group (blue/orange). A 15-second penalty for overshooting the pit stall, or for crew members jumping over the wall too soon.

The new pit procedures changed the complexity of the race. Teams considered it too time-consuming to change four tires since it had to be done under green (at the time, a four-tire pit stop would take roughly 20–25 seconds). For an example of how the rules adversely affected the racing, Kyle Petty ran the entire 500 miles on the same left side tires. Bill Elliott suffered a flat tire early on, and was forced to limp around the track at a reduced pace for two laps before he was allowed to pit, effectively eliminating him from the competition.

By April, the rules were changed. At Bristol, the blue/orange procedure was used only for cautions. The pits were closed at the onset of caution, then opened after the pace car had picked up the leader and the field was sufficiently "packed-up" (that usually took one lap). Once the pits were opened, blue sticker cars would pit the first lap by, with orange sticker cars pitting the next time around. On the restarts, blue sticker cars started on the inside and orange sticker cars on the outside. Lapped cars went to the rear. The blue/orange rule was eliminated during green flag stops. By the next week at North Wilkesboro, the blue/orange rule was scrapped. In its place, once the pits were opened during a yellow, lead lap cars only were allowed to pit on the first lap under caution, while lapped cars had to wait until the second lap.

After a few weeks, the rules were relaxed further. The pit road speed limit and use of the "lollipop" style signboard were the significant changes made permanent (and remain to this day). The second pace car for the pit lane was eliminated. Drivers would be required to gauge their own pit lane speed (by monitoring their RPMs) and officials enforced the infractions with a system similar to VASCAR. The rule closing pit road when the caution comes out also remained in place, as well as only permitting lead lap cars to pit on the first caution lap (lapped cars on the second).

Qualifying was set by the 1991 Gatorade Twin 125 Qualifiers. The top two positions were set by qualifying speeds held for the Twin 125 Qualifiers held on Saturday, February 9, with the top two qualifiers in the session earning the top two positions for the Daytona 500. The rest of the starting was set in the Twin 125 Qualifiers, held on Thursday, February 14 during two races. The top 14 finishers in the first race, excluding the pole position winner, would set the inside row from rows two to 15, and the top 14 finishers in the second race, excluding the outside pole position winner, would set the outside row from rows two to 15. The remaining non-qualifiers would set positions 31-40 based on qualifying speeds from the first qualifying session held on Saturday. If needed, up to two extra provisionals were given to teams high enough in the previous season's owner's standings that did not qualify for the race by either qualifying speed or from the Twin 125 Qualifiers.

Davey Allison, driving for Robert Yates Racing, won the pole with a time of 45.929 and an average speed of 195.955 miles per hour (315.359 km/h) in Saturday's session.^[4]

16 drivers failed to qualify.