Carburettor problems.

[Bob Culver]

Around 1950 when I was a boy my father used to regularly and meticulously check mpg on our RP, often letting it run out (which could then safely do!) Twice a year we motored 200 miles each way to visit grandparents. With two 1950s size adults and two children, driving at 40-45 as suited roads of the time used to attain 50 mpg. Later with a less obstuctive exhaust this worsened to 40 mpg. In my use, with SU and later head and driven flat out everywhere it did 38 mpg town and country.  No attempt to improve the needle.
If the car is entirely stock, not greatly overbored, and attains true 50mph on a true flat road the car (and driver) are doing well. Several contemporary road tests did not reach 50 mph.