Phil, Bill here.
On my various trips out west in my low altitude vehicles, I could never tell the difference. My wifes car, a Dodge Intrepid, was loaded to the hilt with 10 days worth of shooting supplies and vacation luggage. It ran and used fuel as well as it ever has. The Ram that I drive ran the same as it does here in the east. A friends F150 did a good job as well. Naturally it didn't run as well as my Ram, nor get as good a gas mileage, but thats to be expected. (Ops, shouldn't have said that) Heh, heh, heh! I'm told that in the higher elevations of the foot hills and the western mountains where the air is thinner, one doesn't need the same amount of octain to gain the same results as WE do here in the east. A high peak here in so./cent PA is 2,500 ft. That same elevation is a bump in the terrain out west. When you go for a visit, try the 85. Doubt you will need anything any higher. Have a nice trip! Bill.......................