I recently did a family road trip approximately LAX-(I40)-ABQ-(I25)-DEN-(I70/I15)-LAX. The gas price websites (combined with hotel wifi) were very helpful in planning where to and where not to get gas (and how much to get).

Despite living in a mountainous state, this trip was also the first time I learned about gasoline with Octane ratings below 87. As we entered NM, I began to notice the 86 and 85 being sold at gas stations. It took me little efforts online to find out why--driving in high altitudes do not require that much octane.

So on the way back, at Cedar City, UT (5800 ft), we mixed in a few gallons of 91 to the tank of 85 (purchased in CO) so we'll be ready to drop 3k ft down to 2700 ft (St. George, UT) in only 50 miles, and also leave a spare gallon to get to Las Vegas, NV (cheap gas).

Then the surprise: They do sell 85 at St. George!

I didn't remember seeing any <87 gas sold in AZ's high altitudes, I guess because you can easily drive down to sea level without refueling, and so I thought it ought to be the same from St. George, only 400 miles away from L.A. and even less distance to Needles, CA (600 ft).

So who decides what is allowed at gas pumps?