The average price of petrol fell by more than 4p per litre in December, according to new figures.
A litre of unleaded dropped from 107.55p at the start of the month to 102.89p by the close, reducing the cost of filling up a 55-litre car by more than £2.
The average price of diesel dropped by 3p per litre from 109.86p to 106.18p, according to the RAC Fuel Watch report.
But many drivers have been able to take advantage of even lower prices in recent weeks. On December 11 major supermarket fuel retailers cut petrol to under £1 per litre for the first time since 2009, excluding promotions.
The cuts come amid a drop in the price of oil by two-thirds since mid-2014.