The intent of the Energy Policy and Conservation Act of 1975 was to make all passenger vehicles in America more fuel efficient. But auto lobbyists convinced regulators to make a subtle change to the bill’s text. While efficiency standards for cars would be written into the law itself, the standards for trucks were to be set by regulators at the Transportation Department.
In short, because SUVs are built on a truck frame and are considered trucks, they are not held to the same fuel standards as cars.