weird results: octane matters?

[Dean in Milwaukee]

Just got back from a trip to yellowstone park with my 2006 mazda 5 5 speed:
MazdaUSA - Wireless Home

Despite the epa 27 mpg hwy rating, I have always gotten around 30-32 hwy mpg at speeds up to 65 mph. The owners manual specs 87 octane.

This trip going west across south dakota, wound up bucking a 20 mph headwind all the way across the state while traveling 75 mph with the air on and got only 25 mpg. ( worst hwy milage ever for the car)

In the park, I saw that the 87 octane had 10% ethenol and wound up putting in 91 octane premium instead. This was after getting 34 mpg in the park with the 87 octane. ( this car gets really good milage in the mountains for some reason)

This 91 octane tank got slightly over 39 mpg, ( based on when the gas gauge started moving from being topped up. ( about 3.8 gals used, normally sees 115 miles or 30 mpg, but this tank got 148 miles for 39 mpg.)

Intrigued, I used premium for the rest of the trip, ( out to craters of moon nat'l monument in idaho and then back home, ( milwaukee) and got the best hwy milage the cars ever seen, at 35 mpg at 68 mph with the air on. This is better than the previous best of 34 attained by traveling at 60mph with no air and calm conditions across rural indiana.

Even coming back across SD with a smaller headwind and 79 mph I saw 27.54 mpg.

The only logic I can come up with is that at these high load conditions with 87 octane the engine must be pulling back timing a small bit to keep from knocking, ( killing some power and needing more throttle), and doesnt have to with the premium gas. Ultimately, I saw a small fuel cost savings as fuel economy went up a few % points more than the extra fuel costs did so cost per mile was slightly lower using premium.

The big question is that if this car does run better, ( at least at higher speeds) on premium fuel, why does mazda spec 87 octane?

Thoughts anyone?

[taco]

i dont know but i see more ing timing with 93 than i do with 87, ans see a few mpg difference and some power diff, that is in my 09 taco and 07 se cam.maybe the vvt-i or mazda equivalent makes up for it. and uses it to is advantage.

[SVOboy]

Generally they'll spec whatever octane the car needs to live. It could be they have it set up to advance timing for slower-burning fuels and this might actually be to your advantage, but they just didn't want to scare away owners by requiring it.

Or it could be random tank to tank variation

[hat_man]

I had used 93 octane Shell V-Power in my tank when I was driving farther on the highway each trip. As gas got more expensive I switched to 87 octane trying to save a few $$$. My mileage went down 2-3 mpg with the 87 octane. Now I just pay the extra and run the 93. My mileage is back up and the engine seems to run smoother. I'm not a mechanic so I can't explain it but that's what happened. The best mileage I have gotten was when I could find 87 or 89 octane without the ethanol. The "straight gas" delivered great mileage. I can't find it around here any more.

[Domman56]

i always run premium cause with both of my cars i picked up 1-2 MPG from premium gas

[MadisonMPG]

Do you get better gas mileage with premium fuel?

It's a definite maybe.