By the early 1960's speeds were increasing rapidly. In an attempt to decrease speed and, therefore, increase safety, regulations were enacted to limit engine power and tire size. Car designers were forced to look elsewhere to give their team an advantage. The advantage was found in aerodynamics. Not in the reduction of drag, but in the production of aerodynamic downforce.
Most automobiles produce lift.
As the speed increases, the lift force increases and the car becomes unstable. In order to counteract this problem, modern race cars are designed to produce negative lift. The typical family sedan has a lift coefficient of about 0.3, while a race car can have a lift coefficient of -3.00. One can easily see the significant amount of downforce that a race car can produce. There are a variety of methods used to reduce lift and, ultimately, produce downforce.