I've read that it's best to stick with one because of the additives. They help clean, and if you use other additives with it (Say Shell and BP) it will create a new additive, that may not work well together. So your best bet is to get one brand, and stick with it for at least a few thousand miles.

Don't know if that's true or not though.

 

FWIW:
Some of us with normally-aspirated cars designed for 91+ find that we get better steady-cruisin highway mpg with 87 octane! But lower-power off-the-line. This seems to fit with your explanations/points, Gas-Man: 91+ octane contains less energy per gallon than does 87 octane, due to the octane-boosters such as ethanol.

 

That would depend on whether or not the 87 has ethanol in it.

 

87 octane sure does have ethanol in my part of USA (northeast) .

?Typically does 91+ octane have more ethanol than does 87 octane, or are other octane-boosters used to make 91+ octane?

Do the non-ethanol octane-boosters have more or less energy per gallon than "pure 87 gas"?

If these questions are too boring, please feel free to ignore :) cheers!

 

And I thought the higher the octane, the less the ethanol.

 

The ethonal level is all going to be the same, 10%. What you guys are asking about is just octane. Which is the resistance to combustion. Google it. You need a higher octane with more compression and timing. You want it to be more resistant cause as you add more timing (advance it) it causes more heat, well you add more fuel which not only cools it, also gives more BTU or power when combusted. BUt this type of explanation is more suited for engine builders, which I am not.

Back to the gas.

Understand... in michigan. 10% E. Refineries rarely even make 87 octane. They make 84, so that when they blend in 10% of the ethonal (113). Do the math

84 x .9 = 75.6
113 x .1 = 11.3
==86.9 octane

Now typically the 84 is higher so you're good.

Now when they make 93 octane
93 x .8 = 74.4
113 x .1 = 11.3
84 x .1 = 8.4
=== 94.1

That's ruff math anyway, but it shows how they actually make your prem gas that you buy at the gas station that includes ethanol. Then you can see how they manipulate the math(mix) to make 91 or like Sunoco's 94.

The biggest crock is 89. Being there is no such mix at most tank farms. That is blended at the gas station pump with their 93 & 87 tanks. I just don't trust the average gas station owners.

 

HI,, I've got a '08 2.4l HHR 17K miles that has gotten poor fuel economy from day 1. I was in a training class a month ago and this subject came up. One of the other techs mentioned that my running 87 octane was the problem due to the knock sensor "probably" picking up detonation and pulling the timing back. I've changed to 93 but in 3 tank-fills haven't seen any change. Always at Speedway (Marathon in Cincinnati) with newer tanks, where I fill my '05 Lesabre 31mpg and '87 carb'd Accord 30mpg. Any thoughts would be welcomed. Phil in Cincinnati

 

What the tech said is correct. You did not mention the HHR fuel mileage. What fuel mileage are you getting? I average 30-34 (90% highway driving) with my 2006 2.4L (38K on the ticker). Since the cold weather has arrived, I average 30-31. The HHR sits outside 100% of the time. BTW I use Speedway 89 gas.

 

The tech guy might be correct. Try running prem and see if it helps. Wouldn't be the first motor I've seen happen with that. But read up on my posts above. Consider also, trying a branded gas. Marathon, BP, Mobil, Exxon, Sunoco. It could be the amount or "flavor" of additives that aren't working as well for you. Speedway is marathon's generic, FWIW.

 

If you are getting poor mileage it is one of two things..Driving technique or something is amiss with the way the engine is set up. My 2.4L auto 06 HHR has been averaging over 30 mpg on, running 87 octane mostly, for 75K miles. I saw no differences in mpg running 93 octane or running pure gas. Neither, in my opinion, was worth the premium price.