Quote:
Originally Posted by Mad Max
I'm wondering how that reduces pollution, surely if people are on the road longer they're creating more pollution.
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That would only be the case if the vehicle’s fuel efficiency was the same at 50mph as at 70mph. But that’s not how it works.
If you have a turbo-diesel, for example, the turbo charger will kick in around 2,000rpm. Depending on exactly how the engine and gearbox is set up, at 50mph in top gear (6th, in my Berlingo automatic) the engine is revving around 1850rpm and the turbo isn’t operating. At 70mph the revs are in the region of 2,200rpm and the turbo is operating. At the lower speed the car will do about 60mpg, whereas at the higher it’s around 45mpg.
On that basis, a 60 mile journey at a steady 50mph will burn one gallon, or around 4.5 litres, of diesel. A 60 mile journey at 70mph will require about another third of a gallon on top of that - roughly 1.5 extra litres of diesel.
A car that gets from A to B using 4.5 litres of fuel is clearly causing less pollution than one that uses 6 litres for the same journey, regardless of how long that journey takes.