On my 2005 Aviator there is a sticker on the fuel door that says 91+.
I have always used 91 octane.
I did an experiment last week, went on a highway trip, steady 65 mph, no wind, flat road, no hills.
I reset the fuel economy read out for a new average once I was on the highway and got a steady 17.5 mpg.
Later on when I needed to fill I bought some Sunoco Ultra 94, had that 91+ sticker in mind.
After about 10 minutes on the same road with the same conditions, I reset the read out for a new average, it went to 20 mpg and settled out at 19.5 for the rest of the trip, about 60 miles.
So those 4.6 liter DOHC engines do like higher octane and you should take that fuel door sticker seriously when it says 91+.
That motor wants more than 91 and will deliver it's best fuel economy with more octane.
The worst part is I work for another oil company and I don't think my bosses would be too impressed to see me filling at Sunoco.

 

When the engine runs on a lower octane, it retards the timing. Many newer engines use high compressions, or use advanced valve setups to improve the amount of air that gets into and stays in the engine, which has a similar effect to increased compression.

Not all engines work that way, but as a general rule, you should use the octane recommended by the manufacturer. A higher octane will not always help you. For example, the 4.0L OHV in my Aerostar wants about '87 octane. If you run lower it will ping. If you run higher, the engine sounds better, but the gas mileage drops off, and over time, carbon deposits form. My engine is not a high enough compression to justify higher than 89. The newer 4.0L engines are more efficient, and can use higher octane gasoline. But you will get bad economy if you don't use the right fuel.

 

i agree with Bear, the computer has to sence "ping" and if it does not see it, then it advances the timing. if you use hi octane in that vehicle then the milage goes up. if you dont have a car with that version of computer, then it wont help.

 

The 91 could have had more ethanol content than the 94. There are so many variables. The 4.6l in the aviator has a CR right there with the 5.4l 3V off under 10:1 and it runs on 87. I highly doubt its anything more than a fluke mpg wise. The computer will only advance or retard timing I believe 2 degrees either way which is not enough to effect mpg. I'd say your fine running 91. Besides IMO 60 miles isn't enough to get an accurate mpg reading IMO. You should run at least 3/4 of a tank through and hand calculate mpg. Also make shure your stopping the pump at the same number of clicks also to keep it fair.

 

Actually, the engine you have is designed for 93 octane, but uses a knock sensor to allow you to run on 91, which is the highest octane in many areas. Using the lower octane reuces economy, because the timing is being retarded, and the engine can't give you all it has, and the increased advance on the higher octane allows more of the fuel to burn in the engine, increasing both performance and fuel economy.

The computer can advance the timing up to 30° (15° either way). That will dramatically effect gas mileage.

 

Yes Bear, I have read that too.
Someone once told me that the need for 91 octane was a farce and to just put in the 87gas.
What a drop in fuel economy I got! Went from 17 highway to 9. But it didn't ping at all.
The 94 Sunoco I used was 10% ethanol. The 91 had none.
Now to find 94 with no ethanol, I might get a little better economy.