91 is premium at probably 99% of stations here. There are a handful of stations on the front range that sell 93. But as conekiller mentioned, due to less oxygen at altitude, all fuel resists detonation better here. 91 in Denver is more like 94+ at sea level.
Just curious, and would never do this if I got flashed, but because of the elevation, wouldn't it be possible to run a 93 octane tune here on 91 without any water/meth or knock?