Diesel dosen't have to be refined as much as petrol, it's actually much cheaper to brew.
The only reason it's more expensive is due to demand.
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That's bollocks, the law of supply and demand says that if there is more demand a product will become cheaper (overheads spread out more across all the units produced). The reason diesel IN THE UK is more expensive that petrol is TAXATION. I told you, HM government expects to rake in a certain amount of cash from fuel, if we all move from one thing to another they'll just increase the tax on whatever it is we're using now. if everyone ran their car on LPG they'd heap the tax on that, so if we all start using chicken sh!t as fuel they'll heap the tax on that instead (or, more directly, on chicken food, chicken ownership - "feather tax", chicken wire and whatever else is used in the production of this new wonder fuel.)