BTR: Its amazing the amount of people that don't understand which petrol they should be putting into their cars. I've come across so many people that use 91 because its the cheapest.
Using the wrong fuel can damage your engine and actually give you worse mileage than using the correct fuel.
While that is true, most cars sold or imported into NZ will work perfectly on 91 (or less) octane fuel, and for those cars, going to a higher octane rating is literally just throwing money away and in fact will result in (ever so) slightly worse mileage.
Only some euro's and high performance cars genuinely need higher octane fuel. Octane rating is about ignition profile and resistance to knock, not about power/energy/cleanness of fuel as many people think. In fact higher the octane, the less 'bang'. Most reasonably modern cars that will 'knock' on lower octane rated fuels will adjust timing to compensate, preventing any damage at the cost of economy. All the ads on TV about getting better mileage on higher octane fuels are really only true if your car is adjusting timing to prevent knock.