You can easily beat cruise mpg on the highway if there are hills of even slight grades. However this will require more attention than most (including myself) are willing to give. The difference in 18-28mpg g6 4dr 3.5l v6 and 21-28mpg hhr 2.4l 4cyl, both autos, will be minimal if any because of the similar cd, and weight. The bigger engine almost always loses in the city, and the extra torque and probably better gearing (or simply being able to hold a higher gear at a given uphill grade) will help the g6 out on the highway. Do you live in a hilly area??

i'm surprised at the low mileage people.
i've had this thing for about two months, and the lowest the DIC ever went was 23mpg 100% city driving(2.4LT manual with 87 octane). it has 57, 000 miles on it, 17 inch tires(higher rolling resistance), and i'm not afraid to punch it once in a while(although i've been accelerating more slowly cause i love watching the MPG go higher and higher).
i drove behind a big rig(not draughting, really, he was two seconds ahead) for the last hour of a long trip, and reset the DIC. i picked this particular truck, because he had a heavy load. so, when he went up a hill, he would slow from 60 down to 55 or 50. if i drove like that with no big truck in front of me as an excuse, everyone would be honking at me and gettin' all aggressive and angry, so that's why i stayed behind him. driving in this style, the MPG climbed and climbed all the way up to FORTY THREE MILES PER GALLON!!! i couldn't believe it. there were two adults, two teenagers, and a 5 yr. old with all our luggage stacked as high as the headrests.

if i drive at 70-75 MPH, i get around 32 MPG, 34-36 MPG at 60- 65 MPH. driving conservative(keep her in fifth gear as much as possible), i can get 26 in the city.

this thing rocks. i only joined the forum to tell the world(of disappointed hhr owners...*boggle!*...who posted sad economy numbers) how amazing it is.

although i'm canadian, this is an american vehicle(perhaps from a police auction or something). i'm old enough to remember what a mile is, and i love the miles per hour and miles per gallon thing. adds to the retro feel, eh!?

cheers, y'all.

Right on -i like!-....Many of us are surprised at low mileage.

I Can't complain Bought my 08 Last weekend and went to IN for the 4th. HHR has 180 miles on it when i left. I avg 26.1mpg doing 80mph 50% of the time i was on the Express way.

Mileage
 

I noticed a vast improvement in mileage after 6,000 miles, another increase after 8,000 miles. I get lower mileage with the Alcohol (10%) then regular gas. I have the 2.4 H2 auto. I get 24 arround town and 30+ on the highway. If you use the auto start the mileage is lower. I also have the factory 17" wheeles. :nuts: :one:

vast differance in mileage between 89 and 92 octane gas,3 to 4 mpg

89 octane
70% freeway
20%around town
10% bumper-to-bumper traffic/idling
06 2LT

current avg MPG is 29.9

I've hit 30.5

I have not reset my DIC fuel AVG meter in 80+ days.
I drive it 5 days a week as a commuter, + weekends.

if my car got any less than 26, I'd sell it. That's the whole reason I parked my 4000 lb V8 25.8 mpg Lincoln Mark VIII...was for better fuel mileage. If my 2.4 liter 2LT couldnt beat a 14+ yr old domestic V8 in fuel efficiency, it wouldnt be in my driveway.

I've noticed none. Cruise at 60mph, flat road, warm day, little wind, 35+psi tires, no a/c.....36mpg.....for hundreds of miles, 87 octane and the exact same on 94 in a TURBO SS.....

I've never seen any mpg difference with 93 octane vs 87 octane except my wallet get lighter using 93 octane.

Bumped up to 89 octane from 87 and saw a slight mileage increase(which may or may not be attributable to racking up miles on the car-just a bit over 1750 as of today)but haven't detected any performance improvement.Will be trying some 93 soon to see if there's any mileage/performance improvement.