I have a question on octane ratings. I live in Colorado where 'regular' gasoline has an 85 octane rating, I think due to our higher elevations. Everywhere else I've been to in the US 'regular' gasoline has an 87 octane rating. Here 87 is 'midgrade. I know the owners manual says to run 87 in the 2011 Terrain. I've only run 1 tank of the 85 octane fuel that I filled at 1/2 a tank and don't appear to be having any knocking. I assume this gives me an approximate octane rating of 86. What will be the effects of running 85? If I start getting knocking and pinging will using the higher octane rectify the issue?