I have had a Mazda 3 with their 2.0L 150PS manual option (Sport), from new and now at around 32K miles. It has always run as recommended in the Mazda manual on branded 95 RON unleaded (mostly but not always BP). As an HJ fan I finally succumbed earlier this year, ran the tank low and refilled with 99 RON. It then had another four tanks of 99 RON, all BP, before I went back to 95 RON.

I honestly could not detect any difference in everyday performance - the car drives very well and felt exactly the same on both grades. More surprising, after reading HJ's comments for so long, the mpg was the same. I always keep a very detailed record (brimming the tank, not from the computer) and while on 99 RON mpg stayed almost identical to the long term average. I could actually see the consistency with each tankful because I always zero a mileage trip after each fill then look for the mileage at which the fuel gauge drops to half full. Not sure if it is Mazda or my driving but I have long been surprised at how good that is for guessing when the gauge is about to change (it has eight markers rather than a pointer).

It's OK Misar, you're not the only Mazda car owner who's found this. The mpg in my 1.6 petrol version went up by about 3-5%, essentially 2mpg, which could've been due to better traffic and weather conditions than before. Either way, I didn't notice any difference in actual performance.

What may make a difference is if the person changing uses branded fuels with more cleaning additives and if they use the car for mostly short trips from cold (not my driving pattern), so the cleaning effect makes the car run more smoothly on acceleration or get an mpg level more like when it was new.

I bet that the 2.3T MPS version would be optimised for higher octane fuels, but like most Japanese cars, it can easily get by with 95 RON.