Average speed report from 1936

[Denis Sweeney]

I understand the belief that reducing speed is safer but I do question the validity of the claim that a 20mph speed limit will improve air quality.
My modern, a 3 year old VW Tiguan and my not so modern a 22 year old MGF can only run at 20mph in either 2nd or 3rd gears without their respective engines being distinctly uncomfortable and in both cases at much higher engine revs than they can both comfortably run at 30mph in 4th.
I may be wrong, but I would guess an engine running at significantly higher revs will emit greater emissions than it will running at lower revs regardless of speed of travel??