It is very hard question that I am trying to solve. The problem is the largest variation of your fuel consumption is due to the traffic and mainly your right foot applications, so it is very hard to test back to back. I mean if you really love your car, you will run the best fuel like the BP Ultimate, that is what in my RX-8. Higher octane ratio will give a better response and better power for sure. That is quite dramatic for a big car. Certainly the extra money give driving pleasure. Now your question is a economical one. So lets see. The difference between 91 and 95 is 6 cents/L and 98 is 10 cents/L. So it is about $3.60 to $6 from warning light to fully filled. Or in percentage it is 3.6% to 6% now since a tank in Melbourne just reached $100 for the first time! For that 3.6% difference in fuel bill. It is hard to see why not having something nice.... I can tell you the fuel consumption margin between 98 and 95 is very marginal. I have tried a full tank of Shell V Power and BP Ultimate. Ultimate in one off trial will have a consumption as low as 14L around city, while V Power is getting 15L. Now I use Caltex Vortex 95 (not 98) and get around 14.5L for the last tank, with half the tank used on the weekend and 20% freeway. Depends what is easy around your area. If the petrol station is 5km out then you are using $1 to get there. Now that much nagging about money, you are guess I am a typical chinese now :P