https://beyondtheflag.com/2012/12/02/how-has-nascar-schedule-changed-through-the-years/ "The schedule didn’t change too much until 1971 when RJ Reynolds Tobacco Company took the title sponsor for NASCAR and it became the Winston Cup Series. The is known as the beginning of the modern era of NASCAR. The changes made were the season was shortened to 31 races. All races on dirt were removed from the schedule and all races that were shorter than 250 miles. Some tracks that around used by the Sprint Cup Series anymore were Hickory, Greenville-Pickens Speedway, Smokey Mountain Raceway and Trenton Speedway. Those are to just name a few but there were many others."