Why are petrol prices dropping when diesel prices aren't?

I buy most of my diesel at a Shell garage on the A30 here in Cornwall. Last week the petrol price fell to 130.9p, whilst the diesel price is 140.9p. What is going on?

Asked on 3 October 2012 by TB, Padstow

Answered by Honest John
Supply and demand. Using existing refinery technology, only a limited proportion of each barrel of crude can be refined into diesel. In winter, because diesel is basically the same as heating oil, demand increases against supply and so the price goes up. Expensive refining technology could be used to refine more of the barrel into diesel, but that would put up the price.
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