NASCAR stock cars were previously allowed 850 to 900 horsepower; however, this amount was reduced to 725 horsepower in 2015. The lower horsepower is used with a tapered spacer, which is used in the NASCAR Nationwide series. In addition, the 2015 rules include a reduction of 6 inches on the rear spoiler, which cuts down on the downforce for drivers.
The 2015 NASCAR rules also outlined other changes, such as the replacement of flat valve lifters with roller valve lifters, an optional driver adjustable track bar, lower rare differential gear ratios targeting 9,000 rpm, minimum vehicle weight reduction of 50 pounds using ballast reduction and updated qualifying formats for all tracks. The rules have added rain tires for road courses, rear flashing rain light, defogger and wipers.
These rules also outline the testing regulations, which enforce a NASCAR-mandated ban on all team-initiated private testing. The only tests allowed are from NASCAR or Goodyear. In addition, NASCAR no longer conducts a preseason test before the season opening of Daytona 500 at the Daytona International Speedway.
Other changes include a new officiating process, which requires a capture system and data log used in pre-race inspections, as well as automated pit road officiating at race events. In addition, newly approved parts include revised brake clippers, and updated business processes consist of an approval process for new parts and an electronic rule book.