[ainarssems]

Not necessarily, in fact higher octane fuel usually has lower calorific value so will use more fuel to provide same amount of heat energy. But it comes down to efficiency and ignition timing. If the fuel is too low octane engine will be knocking and ECU will retard ignition timing to prevent it, this will reduce efficiency and increase fuel consumption.

If you look at measuring blocks on VCDS for knock retard under different load conditions you will see if there is any ignition retarding happening. Strangely enough my S8 runs equally good on Shell V-Power and Shell 95 with only little retardation at heavy full throttle loads but my m8tes 3.7 would knock and retard quite a lot even at part throttle on 95 and he was never ready to pay premium for better fuel. I put it down to fact that mine has always been run on V-power and it was very clean in combustion chamber when I took it apart. I can only assume that mates had significant carbon build up in combustion chamber causing pre-ignition and knock due to poor fuel used and possibly oil/servicing in the past.

I once filled with Tesco 99 while I was away from familiar territory and almost empty tank after which I had to press gas pedal quite a bit harder than usual and mpg dropped considerably. Got home, checked retard and any slightest touch to accelerator would retard ignition, I am not saying that all Tesco fuel is bad but I must have got a bad batch. Filled up with V-Power after and was back to normal.

So to summarize check measuring blocks for retard due to knock on VCDS, if you get fair amount of retard on 95 you might save some fuel by switching to 98 and reducing ignition retardation, if there is hardly any retard then you probably will not notice any improvement on mpg going for higher octane fuel.