The greatest (most widespread, most innovative, just plain insane) cheating in NASCAR was during the '60s:
"It’s generally agreed that NASCAR cheating reached its high-water mark (or low water mark, depending on your view) in 1966, when Yunick built a Chevelle that looked exactly like one off the production line, but was actually one-eighth smaller, reducing aerodynamic drag and making the car dramatically faster."
http://www.theglobeandmail.com/globe-drive/car-life/nascars-golden-age-of-cheating/article4178760/