Supermarkets have announced cuts in the price of diesel from Wednesday after recent falls in wholesale prices.
Morrisons was first off the mark with news that it was cutting diesel by up to 2p a litre. Sainsbury's, Asda and Tesco then followed suit.
But motoring groups said pump prices remained a postcode lottery, with a wide disparity in what motorists paid.
The AA said that the average price of diesel is 143.39p a litre, having peaked this year at 146.38p.
The average price of petrol is 137.75p a litre, compared to the year high of 140.03p in March.
The AA said: "Although the reduction in the price of diesel is very welcome, the question is to what extent this is reflected in rural areas where it is essential for country business and activities.
"There is traditionally a springtime fall in the cost of diesel as the demand for heating oil, which comes from the same part of the oil barrel, drops off."
Morrisons' petrol director Mark Todd said the cut was the result of "a drop in wholesale prices, combined with an improved exchange rate".
Consumer surveys have consistently highlighted the impact of rising pump prices on household budgets.
Supermarkets also cut diesel prices in mid-March.
A rise in fuel duty scheduled for September was cancelled by Chancellor George Osborne in his 20 March Budget speech.
The AA said that wholesale petrol prices were also falling, adding that it believed this means "a 1p drop in the pump price is on the cards".
In the United States, diesel prices have fallen for six consecutive weeks according to latest figures from the US Energy Department.