Way better mileage on 85 octane (?)

[AzDon]

My 06 Trailblazer (4.2) gas mileage jumps 2MPG immediately after entering either Utah or Colorado (from AZ) and re-filling with a half tank of 85 octane regular with all other conditions being identical ( temps, terrain, elevations, etc)..... If the mileage instead deteriorated, I could surmise that the mountain grades of Colorado versus the flatness of Monument Valley Az account for the difference, but my mileage improvement is in spite of the more challenging terrain.....
Returning to Arizona, my mileage diminishes by roughly the same amount as soon as I get half a tank of 87 octane and erodes further with subsequent fill-ups of 87 returning to the Western edge of Arizona (essentially sea level and hot)
I'm unable to buy 85 octane in Arizona, so I can't compare in identical conditions, but I believe my TB runs better on the stuff....
My in-town gas mileage on 87 octane is usually around 12 mpg and sometimes as low as 8mpg which seems really bad for a six cylinder vehicle......I was getting 22-23 on the steep highways of Colorado..... Very weird!

 

[6716]

Gas Mileage of 2005 Chevrolet TrailBlazer

Search by Model. Search by make for fuel efficient new and used cars and trucks

www.fueleconomy.gov

Select the right octane fuel for your vehicle!

We answer your most frequently asked questions about octane.

www.fueleconomy.gov

Your experience doesn't make sense in light of those two links, so I guess it would have to be explained by something else.

 

[gmcman]

My 06 Trailblazer (4.2) gas mileage jumps 2MPG immediately after entering either Utah or Colorado (from AZ) and re-filling with a half tank of 85 octane

Ummm........I don't think your 06 TB is setup for E85. You may want to check that.

I don't know what components are in your 06 as far as being "acceptable" in handling E85, in terms of hoses and seals....whether or not your PCM can even compensate for E85.

My advice is to not use E85 until you confirm this.

Also, if you have poor mileage, which sounds like you do, how old is your upstream O2 sensor and plugs?

 

[Mooseman]

I don't think he means E85 but 85 Octane.

My guess would be that it's adjusting to the low octane and higher elevation. Although we don't have that octane here, from what I have read, it should never be used in a modern vehicle. Found this:

Gasoline with an 85 octane rating was previously used for carbureted vehicles in high-elevation areas, but you should not use it in your modern vehicles, as it is not designed to do so.

Modern vehicles, with their electronic controls, automatically adjust for higher elevation. And if it's pre-detonating, the ECU will pull back timing to compensate and save the engine.

Another possibility is that 85 octane might not have ethanol, which raises octane. Ethanol has been proven to lower power and MPG so maybe the extra MPG is from the lack of ethanol.

In any case, I wouldn't use it.
I remember years ago I put some 85 octane in my 78 Camaro in Pennsylvania because it was cheaper and it pinged like crazy.

 

[gmcman]

I want to speculate that the reason it should not be used in modern vehicles, is the fact the PCM may not be able to compensate enough for any pinging if it becomes present.

 

[aaserv]

Gasoline is blended differently in a lot of states and the blend even changes from summer to winter. I would suspect the EPA blend in Arizona is closer to the crap they sell in California. Probably just getting better gas in the other states.