Originally Posted by fast murray
When I had a gasser I always ran the 87 that was not ethanol. We used to have 87 which was no ethanol, 89 that had 10% ethanol, and premium which was 91. Now they are mixing 85 with 91 to create 87 with no ethanol which makes it about 30 cents higher and the ethanol blend is now 87. On the other side of our state they sell 85. Steer way clear of that stuff! I inadvertantly filled with it once while pulling our 5er and my 5.7 Ram hated me for it!
The 85 octane should only be sold in areas above 3000ft (approx) elevation - where it is sold as "regular". For an engine without a tubo- or super-charger, the reduced ambient air pressure reduces maximum engine power anyway, so that the full 87 octane is not needed. BTW, you also get better mileage at altitude due to both reduced air resistance and reduced power from the engine.
As was pointed out, the 85 octane will often cause performance issues down at sea level if you are using anything close to full power.
Turbo- and superchargers restore air pressure (and then some), and so need their normal octane gas while driving in the mountains.
Ethanol has an inherently higher octane rating than gas, and so is used as an additive (where allowed) to boost the octane rating. But ethanol has less BTUs/gal, and so mileage suffers slightly. Before electronic variable ignition timing, engines (such as in dragsters) would use alcohol for maximum output in short bursts because of the higher octane.