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I think the worst mileage was actually in the early 1970s—cars were heavier and engines were bigger. In my research for the Riviera Project, folks complain about the fuel mileage until about 1977. The first-generation cars got 10-11 city. By the time the Feds get around to testing in 1973, the range was 8.2-9.8 city (!).

For the Riviera, the worst of all was the example that was also the heaviest ever—1974's model was rated at 8.3 mpg city. After that, manuafacturers figured out how to game the system and cars actually got more efficient:

1975—12 mpg city
1976—12 mpg city
1977—15 mpg city