Here in Colorado, gas stations sell "regular" gasoline which is rated at 85 octane ((M+R)/2). In most of the rest of the US, 87 octane is the lowest grade available. (As noted previously, higher octane fuel can withstand higher compression without detonation.)
According to this article from AAA, the stated reason is that most of Colorado is at high elevation, above 5000 feet (about 1500 meters), and the thinner air tends to prevent detonation. Therefore, in theory, one can use the cheaper, lower octane fuel without problems, and save some money. (85 octane currently tends to be about 3-5% cheaper than 87, which is also sold here.) However, the article also says that a 2001 state legislative report called this theory into question, with respect to vehicles newer than 1984. I wasn't able to find a copy of this report to see the specifics; in any case, engines have advanced considerably since 2001, so it isn't clear whether that research would still apply. Is there more recent research on this topic?
The owner's manual for my car (a 2006 Honda Civic) specifies 87 octane fuel. However, the manual presumably was written for the majority of the customers who live near sea level, and may not have considered altitude effects. So I would like to know whether I can safely use 85 octane fuel without risking problems.
As a follow-up, my understanding is that modern engines detect detonation and adjust to eliminate it, at the cost of some performance. Thus, if I do switch to 85 octane and it turns out to be insufficient for my engine, how could I tell?