I get 17mph whether its on hills or flat ground, it doesn't seem to matter.
As an aside, on a trip to Adelaide a few years ago in convoy (1928 Dodge Tourer, 1929 Chrysler 75 Sedan, 1934 Chrysler drop head coupe, 1936 Chrysler sedan and a 1936 Chrysler coupe) all 5 cars used almost identical amounts of fuel. And of course the US gallon is smaller than the imperial gallon.