Feel free to google. I researched all this when I bought my 2012 Mazda3 skyactiv. You can find plenty of data from mazda about running rich to allow 87 octane. You'll need to do more digging to understand the A:F ECU trimming. I believe this technology was rolled out industry wide when Flex Fuel vehicles rolled out as they can actively change engine tuning (not just timing) based on running regular unleaded vs E85. Most of the links I have from this time period of research no longer work.

There are UOAs on this site proving the fuel dilution goes away when you run premium. The proof is in the pudding, so to speak.

I don’t think it’s just the Skyactiv engines that Mazda richens excessively- my 2011 Fusion was very predictable- let’s call say 80*F the baseline where best mileage was achieved. In fall/winter, it was extremely predictable- with no changes in driving route or throttle application, every 10*F drop in ambient would shave 1mpg from the hand-calculated average. In hot summer, I would routinely knock down 36.8-38mpg on my work route.

Take that down to say 10*F in December, and I would see mileage fall to 29-30mpg, and no excessive idling either- wait 30 seconds for oil to circulate and then proceed on the 7-minute drive out of town before hitting the highway. However, there were no statistically significant changes to viscosity, TBN, or wear metals during cold weather.

I have the same exact car. Glad to see a good report on these engine.

I hope you enjoy your Mazda3 as much as I enjoy mine. Really nice car so far!

FYI: In all my other vehicles, I typically go from 87 to 89 octane in the hottest months of the year. This is just a (old-school thinking) preventative measure but, it does help in my wife's 2007 Madza6 (confirmed observation).

My tank is 13.2 gallons. Last week I filled-up with 10.7 gallons of 89 octane because our temps were hitting in the mid/high 80's and last couple days, we've had low 90's. I now have 150 milles on that tank and the trip computer is not showing any noticeable change in fuel economy. The engine does seem slightly more responsive but, I doubt it's from the difference in fuel. It's probably related to the overall higher temperatures. This vehicle loves warmer temperatures.

I will continue using 89 octane until the next oil change and will note observations as time passes. We shall see if there are any significant differences in fuel dilution or viscosity and of course, fuel mileage will be logged. In the following year, I'll just use 87 all year.

Ray

I know back in the day the Mazda's shared platforms with Ford. Is this 2.5 Mazda one that is a shared design to the Ford 2.5?

Nope it’s all new. Only the displacement size remained the same.

Pretty sure the "Skyactive" line came out in 2010/11 timeframe and that's when Ford and Mazda went separate ways. I had a 2009 Mazda6 that had a 2.5L engine and it was identical to the Ford Fusion engine.