Originally Posted by OLD BOY

You may believe that, but if you recall, I responded:

‘I’d love to know how they worked that out. Did they carry out an experiment?’

Figures may be banded about that someone thinks up, and people are just supposed to believe it without question.

It stands to reason that travelling at 20mph takes longer to travel from A to B than a car travelling at 30mph, which means that engines are running longer, in second or third gear, and car bunching increases the concentration of fumes before they can be blown away by the wind.