87, 89, or 91

91 is the highest I can find around here, unless you pay out the wazoo at a few specialty gas stations.

 

91 or 93. Only because I'm tuned. I assume 87 or 89 would result in pre ignition knock. But I don't know for sure. I'm pretty sure some Untuned guys have run 87. Maybe they'll pipe in.

 

93 only

 

In my 06 HHR 2.4L, I mostly run 87 and after 134,000 mile, the car gets 30+ mpg average. I have tried running 93 (a 2000 mile experiment) and saw no mpg improvement, but did see a lighter wallet. 93 octane is $0.40/gal more expensive. I have even run pure gas which is $0.80/gal more expensive and also saw no mpg improvement.

I will note that the car seemed more responsive using the higher octane fuel, but the car is a daily driver and not a stop light to stop light drag racer.

 

Old Lar. Are you trying to butt in on a SS discussion ?

Anyhow. 93 octane only with my GMTU. Average is right at 28 mpg per tank.

 

93 or 94. The couple times it has had 87 in there, I have noticed a drop in MPG as well as it feeling sluggish. It didn't knock, though (stock settings).

 

On an HHR SS , 91 octane, is the minimum...
Anything less, will cause reduced power, & reduced engine life…

Boost + low octane = death...
The more compression that your engine has, you need higher octane, for a slower burn.
Boost = higher compression

Now, for non-boosted engines, read your owners manual, & try to use the
cheapest, lowest octane fuel, that the engine likes, without ping/knock...

The lower the octane, quicker burn rate, = better mileage...
87 = quicker burn rate
93 = slower burn rate

People that think that, "my baby deserves the best", with premium fill-ups, with an engine that only needs 87 octane,
pay the most, for gas with a slower burn rate, & therefore, get the worst gas mileage, ...

 

E47

 

E47 = appox: 99 octane...

Octane calulator when mixing in Ethanol:
http://www.google.com/url?sa=t&rct=j...lEKlH5xvv8lryw

Octane is really a gasoline rating, as Ethanol octane rating, is just one part of a bigger picture.
Ethanol has a very high latent heat evaporation rate,
this means as the ethanol is made into tiny droplets ,
the Ethanol basically sucks the heat out of the air.
This drops the intake mixture temp like a prom dress.
This process helps prevent detonation, therefore you can use
more timing, compression, etc to build more cylinder
pressure, hence more horsepower (actually more torque).

 

Hmmm

This e47 stuff is interesting.

However, I would hate to go the e47 route and have the car tuned for it, and be stuck somewhere where I can't get any.

I've started running 91 after reading these comments. That's the highest octane I can find around here. I figure it's a small price to pay, and my mileage seems to have increased a bit.