Well, I guess there'll be differences automatic and manual vehicles, but traditionally 30 mph on 4th gear tends to be sweet spot for the lowest consumption and and pollution levels in urban driving (not necessarily the sweet spot ovreall at any speed, of course).
It is likely newly manufactured cars are starting to reflect the trend of decreased speed limits in urban areas across Europe, but older manual cars are far from efficient at 20 mph.
My 400cc automatic bike consistently returns far, far worse fuel consumption figures at 20 mph than at 30. Though of course that might not necessarily be the same for other automatic single gear bikes.