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If I recall correctly, about 10 or so years ago, the pricing difference between 91 RON and 95 RON fuel grades at the majority of Service stations was about 4-5%. About 5 years ago this gap seemingly grew to about 6-8% and even more recently, has grown even wider to about 10-12% at some of the service stations here in Auckland as pictured here where 91 is $1.90 and 95 is $2.16. (Pictured is from the local Abbotts way Caltex, but noticed that Caltex and BP stations all over Auckland have done this, including many in South Auckland)

Is anyone able to perhaps may be shed any light on why the gap between 91 and 95 has in these cases widened and apparently continues to widen (percentage wise)? Thanks.

The price gap between 91 and 95 used to be 5c/litre but has gradually risen in the last 8 years or so to 9c, partly due to an increase in the commodity price for premium petrol. If you see a price gap higher than that, it will be because the service station is discounting regular grade and not premium. See http://www.aa.co.nz/petrolwatch for current national prices - 91 is currently retailing for $2.089/litre excluding discounting. Some brands do discount all grades, so I'd encourage you to shop around.