Im gonna say that on our '08 SS auto, the sweet spot is 63 mph, had it on cruise going from G.Rap to Mack. Isl. & got 37 mpg !!

 

0 MPH with the engine off.

But I like the more serious previous answers too.
It is always nice to know I'm not doing poorly.

 

The guy I bought mine from said he noticed the best from 65-70mph (and it is a 2.4L, 5spd) and NEVER saw less than 27. My next door neighbor has a 2.2L auto and said that there is a steep drop >70mph. In the few tanks that I've run through mine, I'm getting about 27 with 65% city, 35% hwy and trying to keep it under 3ooo rpms for shifts.

Hope this info helps...

 

I came back from Cape Coral yesterday ~180 miles, DIC average speed 53, DIC said 33 mpg, refilled calculated 35.1 mpg. The DIC in my HHR has always read low. 2.4L auto. I'd say that 55 mph is a sweet spot. The car is running at 2000 rpm.

 

I see the best around 60MPH...

 

drafting..........will yield much better mpg.

 

I got my best a tick under 65. 2.4L auto here also. 32.9, but had some hills to deal with too. My actual was 33.6. Close, but not quite what you saw :-)

 

Agreed. I can post some crazy high numbers on my little 2.2l 5 speed HHR @ 55.

 

Reliably the best seems to be between 55 and 60. But the best I ever averaged was a little over 65 with the cruise on and having to change elevation from right at 4000 to over 10000 and then back down to about 4500, 36.9 mpg. With a 2.2l and an auto trans I feel this is very good. I've seen in the high 40's for a while but by the end of that trip it was down to 35.1. Engine speed at a specific ground speed is the biggest factor.

 

Drove from Middletown, Ohio to Columbus and back yesterday...filled up at the bottom of the on ramp to the highway and filled up again at the same station on the way home. Was about 190mi round trip. Avg'd 66 mph (with a stop at the airport). Had the cruise set at 70 all the way there and back. DIC suggested that I got 29.1mpg. My calculator suggested 32.5. Not too shabby for a 2.4L