The 25 Hours of Thunderhill is an endurance racing event held annually during the first weekend in December in Northern California, at the Thunderhill Raceway Park; just outside the small city of Willows.
The race is longest road event on a closed course in the United States.
It has been run since 2003,[1] sanctioned by the National Auto Sport Association.
[2] It is the successor to the Timex 12 Hours of Thunderhill event run between 1998 and 2002.
[3]