In 1952, he switched to racing the American Automobile Association (AAA) big cars, champ cars and stock cars. Luptow switched to stock cars at the behest of his wife, who wanted him to quit entirely because of the danger of open-wheel cockpits. He failed to qualify for four champ car events including the 1952 Indianapolis 500. In the stock car, he won twice in August at the Milwaukee Mile and the Terre Haute Action Track. During the September 21, 1952 AAA stock car race at Lakewood Speedway, Luptow's front axle broke causing the car the flip. He was partially ejected out of the car and died shortly afterwards of head injuries after the car rolled on top of him. Luptow had previously planned on racing at Terre Haute that day, but traveled down to Lakewood as a favor to AAA, who wanted more star power at the Lakewood event.