Keep in mind - - -  - SOME CARS - - - - .  Most cars are designed to run on
87 octane and using the more expensive premium fuel just wastes money.
Mercedes, Corvette, and some performance cars can use the hi-octane stuff
and even require it.

If the manual says 87 is OK, I'd try the 86 and see how it goes, for one
short ride.(or partial tankful). If it requires higher per the manual, you
probably won't want to do that.....It depends how/where you drive. Other
than on a long trip, I might experiment-find out where the engine begins to
get unhappy - if it knocks,ease up and top off with premium......hard
accel/uphill/towing is really where ping happens......40 mph in Kansas
you'll get by with the "cheap" stuff probably. Towing a camper to the summit
of some mountain is a different story.

Keep in mind that octane requirements are to some degree related to
combustion chamber pressure.  Combustion chamber pressure in a
normally aspired engine is somewhat related to ambient atmospheric
pressure.  So, the engine's power ratings and octane requirements are
normally rated at 29.92 inches of mercury and 59 degrees F unless
otherwise stated.

With Southren New Mexico and the El Paso area being at around 4,000
feet, the engine will not see maximum combustion chamber pressures (or
horse power) unless your driving a turbo charged model.

Drop by your local airport and I'm sure you'll find someone willing to
calculate the air pressure at that altitude, or show you a pressure
lapse rate chart.

So, my thought is you'll never notice anything unusual going on at 86
octane and you will be wasting money using higher octane gas, unless,
of course, you have a model which states "For improved vehicle
Performance, premium unleaded gasoline with Pump Octane Rating of
91(RON 95) or higher is recommended..", or you add a turbo charger.

Tom Debski

Really?  Is there an 87 or does it stop to the normal 89 for mid grade?

I recall that Sunoco used to sell a low grade so they could advertise a low
price but I don't know of many people that actually used it.  That was the
old 190 blend.  Regular was 200 and you could go up to the super premium
260.  In my travels I've not seen anything less than 87 for quite a few
years now.

Just saw this message three weeks after most of the thread had played
itself out, hence the e-mail to the OP as well as the post to the group.

At higher elevations like you find in southern NM and western TX, the
lighter air by itself acts to reduce knocking without the need for any
particular octane rating. The compression in the cylinder is lower at
5,000 ft than at sea level, meaning that even lower-octane fuels will
not knock nearly as easily.

It is for this reason that 85/87/91 are the three "standard" octane
ratings in much of the Rockies and Mountain West region, while 87/89/93
tends to be predominant at lower elevations. Your .sig said something
about Arizona, so I will take the liberty of assuming you live(d) in the
most heavily populated part of that state: metro Phoenix (elev. ~1,000
ft.), where 87/89/93 are probably standard because of the lower elevations.

As long as you're going to stay at higher elevations, you should be
perfectly safe to run 85 octane fuel in a car whose manual calls for 87.
A lot of things, from octane ratings to cooking directions for food, are
written with total neglect of the effects of higher elevation (e.g.,
water boils at +203F in Denver, not +212F), and it sounds like your
manual is no different.

The only time you would really need to worry is if you're taking longer
trips outside of high-elevation regions. If you take 85 octane fuel much
below ~3,500 ft., it will definitely start to knock and cause all the
problems that knocking causes. This would be where you'd want to get the
87 octane fuel, e.g. somewhere between Midland and Abilene if you're
headed for Dallas, or somewhere before you drop down into Tucson if
you're headed back for AZ. OTOH, the 85 octane should be fine for a trip
up I-25 to Albuquerque, since that's all in higher terrain.