Back in the 50's and 60's 20 mpg was considered quite good economy. I have a 1953 Lanchester , 2litre , that does 22MPG. back in the 60.s I never had a car good enough to get that, and I mean a Ford prefect, Vauxhall Velox and a Standard Vanguard, all did about 12 MPG due to worn out engines. My present Jag an X 300 4 litre does about 20-25 . but I don't care , after 2 Rolls and a Bentley it seems OK, and I have a Mercedes diesel for cheap journeys.