I have a 2013 Volt with 70K miles on it, most of which have been on the plug. I live in Denver, the "Mile High City" and often go into the mountains with it. The Gen 1 requires 91 octane gas but at higher altitudes, the air is thinner so you can actually get by with a lower octane gasoline with most engines. E.g., our other car is a Toyota RAV 4 hybrid requiring 87 octane but I've run it on 85 octane for a couple of years without any knocking or negative effects. But, GM has been very specific on this issue saying that less than 91 octane will damage the engine. So, is there a Volt owner on this forum who lives at a similar high altitude who has used a lower octane gas in the Volt where it has not damaged the engine? That is, someone with a 2011-2015 Volt.