How do you work that out? Car revs are roughly the same using 3rd at 20mph as 4th gear at 30mph. I find I switch down more to 2nd gear at 20mph, meaning higher revs and using more fuel.
A regular journey of mine has taken me 5 minutes longer both yesterday and today. I reckon that if I'm driving for 5 minutes more I'm using more fuel and creating more pollution.