— Panther Racing Owner John Barnes, for whom Leffler raced in 2004 and 2005, said the racer had a "fierce competitive spirit and a devilish attitude. Jason was a small man with a huge right foot."
From Long Beach, Calif., Leffler made 423 starts in NASCAR's three national series, winning the two Nationwide races and one Truck Series event in a career that began in 1999. He also made three IndyCar Series starts, finishing 17th in the 2000 Indianapolis 500. Leffler won three consecutive USAC Midget championships from 1997-99 before following mentor Stewart's path into NASCAR.
Leffler's last full NASCAR season was 2011, when he ran the entire Nationwide schedule for Turner Motorsports. He finished sixth in the standings that season and hadn't had a steady NASCAR ride since.
The driver had been racing dirt car events most of this year. On Sunday, he finished last at Pocono in his lone NASCAR Sprint Cup start of the year, running only eight laps in a start-and-park ride.
Many drivers included the hashtag "LEFturn" in tweets mourning Leffler. That was his nickname and was written above the driver's side window on his race cars.