Fleets across the country have welcomed the recent drop in fuel costs, which has seen average petrol prices in the UK fall by 6.4 pence in a month.
Average petrol prices fell from 119.5p per litre in mid-July to 113.1 now.
In the past month, the average price of diesel has fallen from 131.5p per litre to 125.5 - a saving of six pence a litre.
The price drops are the first significant reduction in the price of fuel since August 2006 when the supermarkets sparked price war which resulted in a 12.5p and 8p reduction in the cost of unleaded and diesel respectively between August and November.
However, according to the AA, filling up a tank is costing more than it should.
In its latest mid-monthly Fuel Price Report, the AA said that since July 11, when the wholesale price for petrol coming into the UK peaked, the wholesale price has fallen by 18%.
This equates to an 8.5 pence-per-litre reduction in the pump price of petrol, which, argues the AA, should bring the UK forecourt average down to 111.2 pence per litre.
"Whichever way you look at it, many drivers are being short-changed by around £1 a tank when they fill up with petrol in the UK," said Paul Watters, head of AA Public Affairs.
"The AA accepts that small retailers, particularly in rural areas, are likely to charge more for fuel to cover overheads. However, larger petrol suppliers still have some way to go to pass on wholesale savings."
PetrolPrices.com agreed that only a handful of retailers have slashed prices, meaning the gap between the highest and lowest priced fuel is wider than ever - 25p for diesel and 20p for unleaded, compared to a normal differential of 17p.
“This means it is more important than ever to compare fuel prices and shop around,” said Brendan McLoughlin, founder of PetrolPrices.com.
London continues to sell the most expensive petrol in the UK, with Northern Ireland close behind.
Although Scotland has experienced the biggest fall in the price of petrol, the cheapest regions are the North West and Yorkshire and Humberside.
Northern Ireland, the most expensive region for diesel, has seen its average price fall by the smallest margin in the UK.