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Why is "Regular" 86 octane here?

Visiting my son in El Paso, TX. The Manual for his 2009 Sonata says “Use at least 87 octane.” Most places, that means “Regular” gasoline. But hereabouts the Regular is 86 octane, so he’s using the next higher grade. Why are they selling 86 as Regular?

That is news! I have never seen less than 87 octane, is there an ethanol % also?

It has to do with altitude. At higher elevations it is common to have lower octane ratings since the engine operates as efficiently due to the lower air pressure. In the Rocky Mountain west this is not unusual to see at stations.

+1 to @jdmere’s answer. El Paso is at 3200 ft above sea level.

Another +1 to @jdmere’s reply.

Ah! (I should have thought of that.) So, then, even though the mfr “requires” 87, he can actually use the 86 here? Just listen for pinging (or watch for CEL), and be sure to change upward if he is headed for lower altitudes?

Another +1 for @jdmere. I used to live in the high desert of Arizona and 85 octane was regular gas. My vehicles ran fine with 85 but I always filled them up with 87 octane while traveling in lower altitudes.