85 vs 87 ethanol in Denver (Not E-85 but regular 85)

There was an old study that showed that what occurs is that the thinner air helps prevent detonation, therefore at high altitude lower octane can be used (especially in a low compression ratio engine like the Prius). However the study is now outdated and it's no new study has been performed for what occurs for modern cars with ECUs that control the engine behavior (AAA Colorado ~ EnCompass Magazine - Online). I've owned my Prius for about 10,000 miles and had no problems running 85 at altitude, and I assume the previous owner did the same for 60,000 miles.

Something to watch in the Prius with its very long range is that it's possible to drive it from a high altitude to low with 85 in the tank. I wouldn't want to run it with 85 at sea level, so if you go down in altitude, just do an early fill up.

Found this too. There are millions of people in the state of CO using 85 fuel no harmful side effects reported. The vast majority of people don't even think about this and don't seem to experience any problems.
Denver, CO: Is it safe for your car to use 85-octane gasoline at high altitude if the manufacturer's manual states it requires 87-octane minimum? - Quora