I tried to do this on my VW Bora (before it was a write off)
Calculate your mileage on 91 then mileage on 95/98
Using gd1_nz's numbers. If you get only 50km more out of RON96, and based on a 6L/100km, you 'save' an extra 3L. The cost of an extra 3L of RON91 is approx $3.60 (based on $1.20/L). The Jazz has a tank of 42L, so assuming that RON96 is 7c more p/l, filling a fuel tank is $2.94 more than if you filled it with RON91. So you save 0.66c!
I guess that's an extra half litre next time you fill up...
Also, RON96 I don't think has all the fancy dancy engine cleaning stuff as some RON98's like optimax and all that do, so I guess it just comes down to your own personal tests. If you're a real conservative driver and your mileage doesn't very as much between RON's then stick with 91 and save a few cents I guess.
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