Diesel drivers are still paying more than they should be despite fuel prices falling for the fifth straight month in March, according to the RAC.
The motoring experts, who offer an online Fuel Watch service, have analysed data from last month and found that diesel motorists should be seeing "far bigger" cuts. Diesel has dropped to 162.94, saving drivers £15 a tank compared to November of last year.
They say that wholesale prices were very similar through the month and so diesel - costing 160.98p on average in Scotland by the end of March - should be priced closer to that of unladed fuel, which was 145.77p on average.
Asda was found to have the cheapest petrol of the big four supermarkets at 142.69p last month, and Tesco had the lowest priced diesel at 159.58p.
However, member-only retailer Costco is still offering the most competitively priced fuel across the UK with petrol at 137.7p and diesel at 149.8p.
RAC fuel spokesman Simon Williams said: "The fifth consecutive fall in the price of fuel in as many months is without doubt good news for drivers, but those who run diesel vehicles should have enjoyed far bigger cuts at the pumps than they have actually seen."
The RAC calculates that the average retailer margin on a litre of diesel at the end of March was 21p.
Meanwhile, those who drive petrol cars had to pay a margin of just 7p at the pumps to retailers last month.
Simon added: "It simply cannot be fair on drivers that the retailer margin on diesel is currently three times more than the long-term average at 21p a litre.
"While retailers are free to charge what they like for fuel we feel there should be an obligation on those which sell the most, and therefore buy most frequently, to closely reflect what’s happening on the wholesale market.
"If this had been the case then the forecourt price of diesel would have been down to around 152p a litre, even with an above-average margin."
The expert concluded that If small retailers can afford to sell both petrol and diesel for just 142.9p a litre, there's surely no reason why big four supermarkets can't too.
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