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I drive an 05 Civic LX and am considering a move to the Denver area where the regular unleaded fuel has an 85 octane rating. I've been reading a lot about whether to use 87 or 91 octane, but I haven't read much regarding what to do if you live in high altitudes and can only choose from 85, 87, or 89 octane ratings. Some say 85 is fine in elevations over 5000 feet, but others say that newer cars automatically adjust for high altitude driving and the 85 octane may still cause damage to the engine. I'd like to hear about the experience of other Civic owners who drive regularly in high altitude areas and the type of fuel that works best for them.

 
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i would not use 85 at all, even the manual says the lowest you can go is 86 i believe. so stick with the 87 and you will be fine. using too high an octane is also bad. you have a civic with a lower compression engine, it just will not need anything above 87. and yes the car is pretty good at adjusting when the air oxygen content changes. thats what the oxygen sensor and ecu is for. and the reason why your gas mileage in winter is always alot lower. cold air has more densely packed oxygen molecules so the ecu dumps extra fuel to achieve the proper ratio.

 
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Yeah, that's the thing that sucks about Denver (lived there for 3 years), is the crappy octane ratings. It's there for a reason I guess given the altitude. The only Honda I had was my 86 CRX and it ran fine on 85 octane. It also had the 3-barrel carb.

I noticed zero difference between power and mileage on my wife's 2000 Blazer when I used both 85 and 87 octane. It's not a D17 Civic, but that's my experience.

 
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i haved lived in high altitudes my whole life and never had a problem with using 85 in my civic and in my old 91 ranger. my mileage sucks on long trips but thats cause nothing is flat around here.. on long trips i get 36 in my automatic sometimes down to 32 if there is alot of traffic in the mountains. i have ran 85 and 87 a few times without any difference in mileage just a little more money. with gas at 3.19 at home for 85 i will just stick with that instead of paying over 3.20 for 87 and above. gas is only 2.88 here in the springs so its not as big of a deal but i never really drive here. the altitude makes it acceptable to run the lower octane so dont worry about it. if you still needed the 86 or higher here for the low octane they would sell it