AA slams petrol retailers for 'creaming off' cash on diesel fuel He added the premium being charged on diesel meant motorists would have to travel an extra 5,500 miles before they reach the point where the savings on fuel efficiency "break even" with the extra cost of using diesel fuel. In its latest monthly index, the AA said petrol prices had edged higher by around a penny per litre across the country in the past month. Asda remains cheapest on unleaded, but its average price of 131.84pp litre is up more than 2p on the middle of May. The supermarket chain is charging 136.51ppl for diesel. BP is the most expensive of the brands, charging an average of 135.78ppl for unleaded, and 140.40p for diesel. Northern Ireland is the priciest region, with an average of 135.8ppl for unleaded while in London, prices average 134.61p. In January, the Office of Fair Trading cleared supermarkets and oil firms on petrol prices, claiming competition in the sector was "working well." The regulator claimed petrol pump price rises over the past decade were largely due to increases in tax and the cost of crude oil - not because of retailers cashing in.