According to what I'm reading, the Axela is another name for the Mazda-3 we have here in the states. There are two engines offered for the vehicle: 2.0L & 2.5L. Both engines only require "regular unleaded", regardless of transmission type. I don't believe either of these engines are turbocharged from the factory, so there's no forced induction necessitating the use of higher octane fuel.
From your numbers (and also what I read), it looks as though New Zealand uses a different method of rating octane that we do here in the States. Here we use the (R+M)/2 method, which gives us the average of the two rating methods (Research Octane Number or RON & Motor Octane Number or MON). I'd suggest you are using just the RON which is the most commonly used world wide, plus the higher octane ratings than I'm used to. With that in mind, our 87 octane, would probably most closely equate to your 91, and so on up the scale. If 91 is considered "regular unleaded", I'd suggest that is what you should be using.
Something you can do is to run the 91 and see how it runs. Stress the engine a little bit. A couple of ways to stress the engine would be to:
- From a dead stop, take off in a spirited way ... IOW: get on it ... but not so much you make the tires break loose
- While travelling down the road, place the vehicle in a higher than normal gear (if manual shift). If you are going 25kph down the road and would normally be driving in 3rd gear, put it in 5th gear, then get on the go pedal a little harder than normal
In both cases, you'll be listening for the dreaded "ping" or "knock". If you hear this, DO NOT push the car any further or harder. You'll want to put the next higher grade in at the next fill up and do this all over again. If you experience it at the next grade, then fill up with the 98 after that.
I believe you'll be able to use the 91 octane with good results, though, as that's what it seems your car is supposed to be using.