Best possible mileage?

My mom was the gas mileage champ, she had a 66 Belvedere ll tree speed /6, she would get in it turn the key back out of the driveway and go to work, work was les than two miles away with one stop sign that got the same treatment every time, stop first gear hafe way through the intersection the second then it was full on dash shake for five seconds or so third was soon after with the same dash shaking now she was book’n maybe 20 mph always worried about the cop at the park checking speed. She drove like that year round and for the three years she had the car before I started driving she never got less than 20 mpg and would get mid plus on the highway. Her next car was a 72 Gremlin X 6 with a 3 on the floor she averaged in the 30s with that.