Good info. We've had ours for just under two weeks and have put on 600 miles. I've been using 87 and averaging 25 with mixed city/freeway. No rough idle, no pinging. Runs fine. Next couple of tanks will be 93 octane, then we'll see what happens. If I gain 2mpg avg, then it'll be worth the extra money.

Our Cadillac recommended 93, but could run on 87. We ran mid-grade (89) in it most of the time. It ran smoother than 87, though no difference in mileage of performance with the higher grades.

 

Call me a fool if you like, but I've been paying the extra for the premium.

I've only had my HHR for two months, but I've tested my HHR out with both Premium and regular gas. So far, the only difference I've noticed is that when I'm running on regular the fan will switch off or slow down for a split second while the engine is idling, I have yet to notice it do this while running premium.

 

Update: Just finished the tank of 93 and didn't noticed that much difference from 91. I do feel a little bloated this morning though... I think that has more to do with the White Castle I had late last night. FYI

 

I had been running 87 octane in my 06, 2.4 since I bought the car new. In the beginning of 08, I started using 93 octane.. for about five tankfuls and saw no difference in gas milage, but a difference in what a fillup cost. I still averaged slightly over 32 mpg with either. Now that gas is running ~$3.50/gallon for regular, I'm staying with 87 octane.

 

The only time you may see a difference in performance is in a warm to hot climates, like the summer. This is when the air is less dense and the engine has more load (air conditioner) and has to work harder. Then, only under heavy throttle.

GM powertrains have an octane sensor and the ECU uses this information in part to manage fueling and timing look ups.

Look at your owners' manual, GM recommends 91 octane in order to achieve full performance.

I worked at GM powertrain for many years designing engine controls, so if you want to know anything just ask.

I run 87 in my 2LT and 93 in the SS cause I always want full power :).

 

Possibly it's just my imagination, but when I fill up with 87, I notice a loss in pick up and gas mileage around 24.5 MPG. However, last night I filled up with 87 after adding a bottle of Outlaw Octane Booster (available at Advance Auto Parts...it was on sale here in CT for $0.96/bottle) and noticed a considerable difference in performance and gas mileage.

After traveling home, my DIC read that I was getting 26.9 MPG, with the 27 MPG barrier being breached this morning as I took my fiancee to work. I might switch to the better octane rating as I can get gas cheaper when I cross the border to MA each Tuesday.

 

I have an 07 2.4 and run nothing but 87 Octane. I haven't heard any knocking and my combo mileage is around 27/28 mpg.