Research from the group has shown that a 150-strong diesel fleet with vehicles on a 90,000- mile lifecycle could save £138,603 by filling up at supermarkets rather than at other stations.
The saving for a petrol fleet would be almost £190,000 for the same mileage per vehicle.
Figures from the AA show the average UK price for a gallon of unleaded is £4.36 and £4.46 for diesel. At supermarkets, the comparable prices are £4.31 and £4.41 respectively.
Tony Hulatt, managing director at fuel management group CLM, said: ‘There are some very considerable savings in fuel costs to be made by switching from petrol to diesel or simply persuading drivers that the best option for the company is to buy all fuel on supermarket forecourts.’