95 vs 98 Octane - Fuel Economy Difference

[Rodge]

I've typically had a strong bias toward using 98 Octane fuel in my high performance cars over the years despite the fact that most if not all are rated to run on 95 Octane fuel so out of curiosity I took a bit of interest in a fuel economy test I came across the other day that the N.Z. AA ran comparing the two.

They went about it in a fairly exacting manner, (happy to unpack their methodology more if requested too), but basically they ran a Subaru WRX rated to run on 95 fuel completly empty till it wouldn't start and then put 15 litres of 95 Octane in it and drove for 170 klm's until it completly stopped and couldn't be re-started.

Then they repeated the process with the same car, same drivers, same day and driven over exactly the same route with 15 litres of 98 Octane fuel and you would think with the significantly higher octane it would have more energy content and have gone somewhat further to justify the premium price 98 Octane sells for but it ran for only 1 kilometre more at 171 km's. In my opinion this is probably well within the margin for error even in a well disiplined test like this so the significantly more expensive 98 gave no meaningful distance advantage.

As an aside - (Note in their previous issue they'd compared a car designed to run on 91 fuel using regular 91 Octane and found it gave a 7% distance advantage as compared to using an E10 91 Octane fuel, so even though the theoretical difference in energy content between an E10 91 fuel and regular 91 petroleum is 3.3% they found a 7% fuel economy advantage in using reg petrol, now that's what I call a clear result !).

Now I know that 98 gives you a bit more kick up the top of the rev range but over winter when there's often very limited traction is there much point in using 98 Octane fuel in a car designed to run on 95...thoughts ?