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2008 Expedition What Octane at 5,280 feet elevation?

5.4L. V-8

Ford says 87 octane

85 is often 10 cents less.

I’m told that 85 is fine at this altitude?


Thank you.

The density of the air changes as you go higher up in altitude. Most gas stations will have octane according to their location, so 85 will be fine. If you’re going up and down the mountains a lot though, I’d stick with the 87


Except for emergency runs, I drive and accelerate very gently.
Even during emergency runs I am as gentle as possible.
Have never heard any knocking.

Why does the Ford manual read:
“Your vehicle is designed to use “Regular” unleaded gasoline with octane rating of 87.
We do not recommend the use of gasolines labeled as “Regular” with octane ratings of 86 or lower in high altitude areas”.

Stick with what Ford says. They have done their homework. Why? I don’t know, but I know I due use what is recommended in my car (cetane not octane) and if I had the Ford truck I would make sure I had 87.

It comes down to one thing. Even if they did not explain it, they have a reason for telling you to use 87.

Probably because the ECM has a barometer (altimeter) built into it and the engine gets “tuned” for the higher altitude automatically to maintain as much power as possible. This probably includes advancing the spark to the point mild detonation MIGHT occur. But at Denvers 5200’ elevation, the cheaper 85 (posted, you don’t know what it really is) octane fuel will run just fine…I think they start selling 85 octane regular at locations above 4000 feet. At 5200 feet where you are, the risk of detonation should be gone…

It’s interesting to note the oil companies tack on an extra .10 cents a gallon to fuel they sell as “regular 87 octane” in areas below 4000 feet…In Denver, which gets its fuel from many sources, you really don’t know what octane you are actually getting…

Yes, I wondered about the barometric pressure device.

Is the fuel not tested to ensure at least 85 octane? (R+M)/2 method.
Today, here:
85 $2.35 = 0.0276/ octane point
87 $2.45 = 0.0281
89 $2.55 = 0.0286

The only way I thoughthis mattered is if it is supercharged or turbocharged.

Now you have the manual? In the oil change post you didn’t have a manual. Are you totally pre-occupied with this truck or what?

Get snow tires, studded if the roads are icy. Change the oil as the monitor indicates or every 5,000 miles whichever floats your boat. Use any oil you want, they all do the job. Stick with the Ford manual as per oil weight and fuel requirements if you expect Ford to honor any warranty claims. If the vehicle is off warranty use whatever oil and grade of gas you want. If the motor has a knock sensor lower grade fuel won’t hurt it. If it doesn’t have a knock sensor then the lower grade fuel could knock when you are in the uphill grade of a mountain and you might blow out a piston.

You can’t possibly be prepared for every contingency and weather condition that may arise. You can be prepared for the expected conditions in your locale. I think you are doing that just fine.

"Is the fuel not tested to ensure at least 85 octane? (R+M)/2 method. "

You can be sure it’s at least 85 octane. In many cases, it’s probably 87. Octane is very rarely if ever tested. Very few labs have the equipment to do it…

I drove my 1978 Oldsmobile Cutlass from the midwest through the mountains of Colorado a couple of times. At low altitudes, the engine had a trace of spark knock on 87 octane. Up at the high altitudes, I ran it on 85 octane without any spark knock. When I would get down to lower altitudes, it would sound like a marble tournament on 85 octane. I would buy higher octane premium gas and let it mix in to eliminate the spark knock. This engine is a carbureted engine and had I wanted to do so, I could have advanced the timing at the higher altitudes for 87 octane. However, the timing would then need to be retarded for lower altitudes. Today’s cars set the timing automatically. I think your engine will work fine at the high altitudes with 85 octane.

With carburettors, high altitude makes the fuel/air mixture go rich and a rich mixture is less likely to detonate than a stoiciometric mixture, so going to high altitude not only reduced air density, which by itself makes you need less octane rating, but also compounded the effect with a rich mixture.

It probably is, but if you use this truck for emergency runs, then don’t you want it performing at its best?

You are right.
Would 87 octane get a little better gas mileage than 85 octane, so the extra cost would be more worth it?

I heard Tom and Ray speak about using the lower octane fuel at higher elevations.
If it makes no difference, why spend the extra money?
We (the volunteers) pay for everything ourselves.

Thank you.