Most races in the National Association for Stock Car Auto Racing, or NASCAR, start with 43 cars because of a rule adopted for the 1998 season. Placement of the cars within the starting grid is based on the drivers' performances in the qualifying rounds.
In the early years of NASCAR, there were no limits on cars at some tracks. Limits were instituted beginning in the 1970's based on track length. Forty cars was the maximum field at large tracks while small and intermediate tracks could have only 32 cars at the start. The limits increased to 42 and 36, respectively, in the late 1980's.
If more than 43 drivers attempt to qualify for a NASCAR race, only the top 43 drivers after the qualifying rounds enter the starting grid.
As of November 2015, the last NASCAR race that started with less than 43 cars was on June 28, 2014. Forty-two drivers started the Quaker State 400 in Sparta, Kentucky.
The previous race with 42 cars was in the 2001 New Hampshire 300 in Loudon, New Hampshire. Forty-three cars had qualified for the race, but the September 16 race was postponed after the terrorist attacks on the World Trade Center and Pentagon. By the time the race was conducted on November 23, one team had ceased operations, leaving 42 cars in the starting grid.