There are several graphs floating around showing the fuel consumption at 30 compared to 50 with different gears. It depends on your car and the gear used if 30 uses more fuel than 50. If your car uses more fuel for slower speeds and for such a common speed as 30, the manufacturer is an idiot. There are so many 30 zones in Europe that it really is not an argument against them that a car manufacturer can’t build proper cars.

And since we are also switching to electric cars, that problem will go away in the next decade or two.