Another Octane Question

2017 Mazda CX-5 GS
I know the subject has been discussed over and over, but as a brand new owner who lives at 8000’, I have a question. 2017 CX 5 GS, the manual says burn regular fuel but also says 87 octane. At our elevation, regular is 85 octane, and 87 would be mid-grade. Even when we travel down to 4 or 5000', about as low as I ever drive, regular is 86 octane. My understanding is that because of the lower air pressure at these elevations, the octane ratings are adjusted down.

I've only had the car a couple of weeks, but it seems to be running very well on regular (85), and getting over 30mpg in mixed driving, even up and down the mountains. It seems happy, and it can't be overly retarded and running that well (I think), but I certainly don’t want to damage the engine either.

I guess I'm just having a hard time wrapping my head around a 13:1 compression engine burning regular fuel.

What do you think, at these elevations is 85 octane appropriate?

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2014 mazda cx-5 touring FWD
It's my understanding that it's not a normal 13:1 'static compression ratio' but what's called a 'dynamic compression ratio'.And that the valves are open rather late into the compression stroke,so I don't really know.How's that for an answer.
Can't help you on the octane but if it's peppy and gets good mileage then the knock sensors must not be pulling timing from detecting preignition.So maybe 85 is good at your altitude.
I do know mine runs better on 89 than 87 at this 900' elevation and that's what I pay extra to feed it.Try and experiment a little with different octanes and see what the results are.
2019 CX-5 GTR

What do you think, at these elevations is 85 octane appropriate?

Short answer - Yes.
A bit longer answer - since the vehicle is operated at higher elevations for the majority (most) of it's duty cycle this will cause no harm. If and when you go to lower altitudes (less than 3000 ft) I would recommend burning the lower octane fuel out and then using the higher octane. A simple day trip don't worry about it, extended stay / travel - yep make the change.

some more reading:

oh and this surely is NOT a new question:

what does the science say?
2010 CX-9 GT
The air pressure above 8000 feet is about 75% of what it is at sea level. The engine is compressing a much less dense fuel/air mixture, so the peak cylinder pressure and temperature are low enough to not predetonate the fuel/air mixture.

As long as the engine isn't pinging, you are fine.
Occupied Calif.
2019 CX 5 GT-R
I have a 2019 CX 5 with the 2.5L turbo engine. I live at 6000 feet elevation. I run nothing but premium gas, which is 91 octane here in California. But I also don't do a lot of driving since I retired, averaging about 300 miles a month so the cost of the premium fuel doesn't bother me.

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