Quote:
Originally Posted by Chris
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.
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Plus the air resistance is proportional to the square of the velocity. So the drag at 70mph is roughly double that at 50mph.
In any car with a fuel consumption gauge it's easy to see the difference for yourself. Drive at 50 on the motorway and watch the mpg go up.