When you go to many car garages you have the option to use their regular fuel or premium fuel. The premium fuel can be quite a bit extra, recently I paid an extra 10p per litre for it so I could compare the difference. Although I do seem to have had slightly more miles to the gallon, it still did not quite cover the additional cost I paid for the fuel.

The fuel is also supposed to be better for the engine components such as the fuel filter, meaning it could save you money on repairs / breakdowns but this is very hard to measure.

The only way to test it really is to put a fuel in your car and clock the mileage to get out of it until the light comes back on. Then fill the car up with regular fuel and do the same test to compare.