Sorry, this has nothing to do with XFs....other than I'm too chicken to take a car with no spare tire on a long trip. Took the old Chevy HHR (manual calls for premium just like the XF) on a 2,200 mile journey from KC - Sioux Falls-Badlands-Deadwood-Rushmore-Crazy Horse-Colorado Springs-Garden of the Gods......

In those high altitude places they sold 85 octane CHEAP.......so I bought.....to my shock the HHR ran GREAT.....& got incredible MPG........any idea how this is possible? When the air is thin & oxygen low....are these newer cars able to sense & adjust somehow.....never dreamed the HHR could get 32 mpg going up and down mountains????? Even when we could do 80 mph on highways, never pinged once.

It normally gets like 25 mpg driven locally with 91 octane fuel.

 

One possible explanation is this. The octane rating is the minimum that the particular grade can be. In this case 87, but working in an oil refinery I am aware that there are times when the regular goes out the door with an octane level the same as premium.

 

Your post reminds me of my engines class professor telling me about different altitudes and how a car on one octane can run.just as well on another octane , I guess maybe at 85 you hit the degree quicker and because of the less oxygen you don't get the pings... Never understood his explanation because I'm more hands on type of person but what I did understand is if you go back home you must continue to use what you normally use at your elevation

 

I was curious about this so I did a bit of googling since nothing is going on at work. Came across this, hope this explains a bit.
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Great answer!!!! THANKS!!

 

Computers are amazing things and the ECM in your car simply adjusts the fuel/air mixture accordingly. The ECM will also take care of timing (pinging/backfires) etc, to ensure your engine performs the best it can with the given conditions. Many high end car owners that know full well that their car "requires" high octane gas, simply ignore the need and put in cheaper, lower octane gas, knowing that they will likely experience little to no effect in the cars drivablility.

That said, I drive my '82 Volvo with manual choke / Stromberg carburettor quite often to events that have me driving at higher altitudes. With no computer on board, the lack of power is very noticeable as the carburetor starves for air....nothing you can do other then re-tune the carb, only to get back to your "normal" altitude and discover you need to re-adjust again.

 

I just couldn't resist trying the 85 in the HHR......as of Sat. 9/8/12......it was running only $3.53 / gallon in SD, WY & Colorado......Premium here in Kansas City > $4....these are 90-91-92 & sometimes 93 octane grades......which is what I normally use in all the vehicles. Premium in those states was running 30-50 cents higher per gallon than the 85.