87 or 85 octane

I just moved from SE Michigan two weeks ago and am wondering about octane ratings. I have a '96 4.0L HO XJ and a '95 2.5L YJ. In MI, 87 is the lowest octane you can get, 89 midgrade, and 93 or 92 premium. Can I use 85 since we are so much higher, or should I keep using 87, midgrade here in CO? I think the XJ owner's manual says 87 is the lowest. Just wondering if that is what you all run?

Why the hell didn't I move here sooner? TIA Brad

The owners manual said it was designed for 87, so that is what I have always used. The wife's VW Jetta gets the 85 :laugh:

You can safely use the lower number at altitude, and CO is definitely 'altitude'. I almost always run 85 Octane here; when I was doing the daily 130-mile commute from Littleton-Loveland I did a comparison with 87 and 85 by filling them up every morning at the same pump for a month. The only thing 87 did for me was help empty my wallet faster. This makes sense when you realize that the octane rating is a comparison of how rapidly the fuel oxidizes and burns, and is not a measure of energy.

Edited to add:
If it knocks with 85, move back up to 87 but it really shouldn't do that...

Good to know guys, especially since I have seen up to a $0.15 difference per gallon. I will see if I get any spark knocking and then go from there. Ouch, that is a serious commute, Loveland to Littleton Jim.
Ran my first moderate trail out here on Saturday (Caribou Creek). Definitely a little different than the MI mud bogging I was used to. Rocks and dirt are much more fun than mud, certainly on a regular basis. Thanks again.