In the 1970's there were a lot of cars with high horsepower.
Horsepower decreased after 1980. In addition, the number of cylinders also decreased, pointing to a correlation.
Mileage increases after 1973, possibly because of the oil crisis in 1973.
Lower weights starting in 1978 which shows up in mileage, engine size, and power but the acceleration increases.
In 1971 there is a spike in mileage, compared with mileage in 1970.