Some states have different levels. In Colorado 85 is common. I assume it has something to do with the air pressure and oxygen content of the air perhaps?
One of the reasons for this is that for a NA (normally aspirated) engine, i.e. not turbo charged or super charged, the lower atmospheric pressure results in a lower compressed pressure (versus sea level) as the piston rises toward the top, which means a lesser chance of pre-mature detonation (knock) due to pressure, which is what higher octane prevents. So therefore you don't need as high octane at higher altitudes.
Unfortunately that isn't true with turbo or super charged engines, which are more and more popular. I really miss the 93 (and even 95) octane fuel I could get in Boston. I use a octane booster in my car here in Colorado to help offset the lower octane pump gas, and allow my turbo charged engine to make more power (by not having to pull timing to accommodate the lower octane gas).
85 is functional at elevation, but definitely still not preferable at elevation.
Works best with carbureted engines, which are obviously few and far between these days. You shouldn't put 85 octane in a modern fuel injected engine period.
The method of fuel metering the car uses does not necessarily have any connection with what octane you should use. It's all about the engine design. If we are going after rules of thumb, I would say that if the car is turbocharged, it will most definitely run better with higher octane. Although my turbo VW can run with regular fuel, it will just change timing and make less power.
I have a 78 Fiat with a carburetor however, that definitely needs 91 or higher.
VW is seemingly, in my experience, an outlier. Their small displacement turbocharged engines will run fine on 87, but really like 91 (or E-15 88). My '14 Jetta with the 1.8T got better power and fuel economy on "the good stuff."
