RAC research has found that, despite the wholesale price of diesel being lower than petrol during June, motorists still paid 3p per litre more at the pumps. The RAC's fuel watch report for June showed that current wholesale prices for diesel are 1p to 3p per litre lower than petrol, despite average forecourt prices being 120p per litre _ 3p more expensive. Whilst some retailers have attempted to close the gap between petrol and diesel on the forecourt, very few have adjusted the price to truly reflect wholesale costs. Indeed, the vast majority of retailers have opted to maintain a substantial gap between diesel and petrol forecourt prices. Analysts believe that this is due to the very thin margins on petrol, and that retailers are continuing the much higher mark-up on diesel in order to compensate. Simon Williams, RAC fuel spokesman, said: 'The retail price of diesel has almost become a taboo subject despite 2.4 billion litres being sold in May, compared to just 1.5bn litres of petrol. 'While there are twice as many petrol cars on the road, the increase in diesel usage shown in Government statistics and the new car sales figures which show more than 50% of cars and light commercial vehicles leaving the showroom are powered by diesel, confirm that we are increasingly relying on diesel for both business and private use.' The report shows that according to wholesale prices, the pump cost of diesel should drop by around 5p a litre in the next two weeks, though the lack of previous movement seems to indicate that this is unlikely. Mr Williams added: 'Essentially, what is required is a fundamental re-balance of pricing in the retail fuel market.' 'We need greater transparency and a fairer pricing model for both petrol and diesel.'