Some towns are charging far more for petrol and diesel than others and there are even "pump price dogfights" within some cities, the AA said.
It reported that the surge in UK petrol prices had slowed but the current average price, at 137.52p a litre, was still 1.74p a litre higher than a month ago.
The AA said: "The average hides the Jekyll-and-Hyde nature of UK fuel pricing, where prices in some towns have gone up substantially more - some places passing on the full 5p-a-litre lift in wholesale prices during early July.
"This has forced drivers in communities of tens of thousands of people to pay above-average prices for the cheapest petrol locally, even at the supermarket. Cheapest petrol at 137.9p a litre is common in many small market and coastal towns."
The AA went on: "The presence of a competitive retailer is dividing some cities. In Exeter (in Devon), a supermarket and two non-supermarket retailers on the west side engaged in a pump price dogfight at the start of this week, charging 132.9p for petrol.
The AA said the average cost of diesel has risen from 140.24p a litre in mid-July to 141.87 now.
Diesel now costs nearly 1.5p more than a year ago.
Regionally, London has reinforced its position as the area of the UK with the cheapest average price for petrol, at 136.9p a litre. This is 1.5p lower than Northern Ireland (138.4p) which is the most expensive region.
Scotland, averaging 142.7p a litre, pays the most for diesel while the East Midlands shares the cheapest slot with Yorkshire and Humberside, both averaging 141.4p.
AA public affairs head Paul Watters said: "Recent wholesale price falls and evidence of individual non-supermarket retailers undercutting the superstores suggest the UK fuel price climate is cooling enough for some welcome respite for drivers.
"It's probably too soon to expect a price war to break out, but a bit of late summer pump relief for holiday motorists wouldn't go amiss - so long as it isn't restricted to towns with competitive supermarkets."