You don't have to get out your calculator to find out which fuel will be cheaper for you.
What Car?'s Real Cost is the worked out from the latest fuel prices, and takes into account all the other costs of motoring (such as insurance, tax, servicing, likely discounts and depreciation) over three years of ownership.
Real Cost assumes an annual mileage of 12,000, so if you spend significantly more or less time on the road, you will need to do your own sums. Generally speaking, diesel savings will be bigger the more miles you do.