2008 Pontiac G8 GT: Fuel Economy Update

Hey hey, Edmunds finally hit the EPA estimate for the highway!

Also, have you been able to detect the cylinder deactivation?

@rayainsw

thanks for posting that, it's great to have another real-world number (presumably for someone slightly less 'eager' as the IL crew). Pretty decent numbers for a big 'ol V8

sharp looking car. Wonder how the real world mpg compares with the V6, it would worry me that it is a heavy enough car that it wouldn't be much better. I wish they'd give this the Direct Injected engine that the Camaro is getting along with the 6 speed A/T for the V6.
Great deals to be had on these right now. Could have ended up with one recently but the salesman talked me out of it. He just wouldn't stop talking about how bad the economy was and how just about everyone was going to be out of work soon...

Yup, not bad at all. GM manages to keep fuel efficiency in mind on all their vehicles. My friends V8 truck without cylinder deactivation was able to manage over 21 mpg on the highway.

Wow, I love the G8 just look at it, sitting there; begging to be driven! You Californians don't know what crappy weather is! Were getting up to 6 inches of snow here in Chicagoland. And I had to tool around today in my Mazdaspeed 3... with summer tires!

Decent MPG for a honkin' V8. I love how the front fenders swoop down over the fender arches -- reminds me of the way BMWs used to be.

Those numbers are almost identical to mines. Honestly cannot complain. And I have close 20k km's on mines, roughly 12k miles.

Oh and btw, that rain is nothing, you should try this thing in the snow, exciting!

Great fun in the snow, though snow tires are a must(Nokian's are amazing), all season just won't do for Manitoba winter's. The Stabilitrack works great, though there is still a small slide effect for U-turns, but when you turn it off you can be doing 360's quite quickly.

From a Southern California resident, I agree this rain is nothing. I'm always embarrassed with how excitable my fellow Southern Californians get over a small bit of rain. They can't drive in a light drizzle without spinning all over the road. Any bit of rain gets a "StormWatch: 2008" and dramatic theme music on the news. It is really, really sad.

I'd much rather drive in rain then the 4-6" of snow we got in Niagara over night. Getting to work this morning was a mess.

not bad at all for this level of power. About the same as the average V6 crossover.

Two more data points, for those playing along at home:

Point 1:
I filled the tank early this AM.
Traveled 296 miles since filled.
Gage showed just under a quarter tank remaining.
“RANGE” showed 87 miles.
Fill took 14.0 gallons of premium – at $1.86.9/gal.
That is 21.1 MPG.
DIC showed 21.7 MPG.
Approx two thirds of this tank was a portion of the Interstate leg driving home from my Florida trip = 65 to 73 MPH.
The other third was my usual commute, in & out of MidTown Atlanta.

Point 2:
As soon as I filled the tank, I drove my typical inbound commute.
Warm engine, A/C off.
This is approx 18+ miles of Interstate, starting at approx 5:45AM – so reasonable traffic – no cruise control – speeds generally 60 to 75 MPH.
When I left the Interstate, for the final 3 miles or so of surface street driving, the DIC showed 22.4 MPG.
Driven identically, under the same conditions, my Corvette’s DIC would show 28 to 29 MPG.

Car: 2009 G8 GT – just passed 1,000 miles.
- Ray
Hoping for some improvement, once she is broken-in . . .

= = =

I took my first trip in the new G8 GT “ex-urban” = outside the Atlanta metro area last weekend.

I drove a total of over 700 miles.
My drive, each way ( Duluth, GA to the Fla. Panhandle ) is almost exactly 350 miles each way. One ‘leg’ = 3 hours is approx. 200 miles of interstate, with limits varying from 55 in a few construction zones to 70 MPH. The other leg is approx. 150 miles, also 3 hours, because it is primarily speed limited to no more than 55 MPH, and includes a significant amount of 2 lane, through small towns & around Tallahassee - with even lower limits.

The V8 is still quite ‘green’.

In 2 fills ( the first = approx. one half tank and second = approx. three quarters ) each averaged between 22 and 23 MPG, calculated by Excel. [ I had reset the MPG several times, so no verification of DIC display accuracy, yet. ] I did run some errands around where my friends live, and even ran 12 or 15 miles or so in & around Tate’s Hell – on fairly smooth but dirt roads, at 15 to 25 MPH, but otherwise, these were ‘easy’ miles & relatively good conditions, from a fuel economy standpoint.

At one level, I find these results somewhat disappointing. As I have written here, I expected that AFM \ DoD would allow ‘real world’ cruising MPG to meet or exceed the EPA number of 24. This trip, I was in no great hurry & did not cruise at much above “2 or 3 MPH over the limit”. I tried no ‘hypermiling tricks’, but drove conservatively.

The temp was moderate and A/C was off for most of the time.
I kept the gear lever in “S” – not “M” to insure that AFM would engage whenever appropriate.
I utilized WOT ( or nearly ) only a few times during each tank’s run – typically to merge onto Interstates.
Over half the driving is over fairly level terrain.
I used cruise control wherever safe.

I was expecting higher numbers.

I have driven the exact same route about a dozen times, and this is the fifth car I have driven to my friends’ house. I had expected 25. 22.5 does not thrill me. My Corvette returned just over 28 under near identical conditions – even when nearly new.

Tests at steady cruise on straight, level deserted roads:
Cruise set at 60\61 = 25-26 MPG.
Cruise set at 62\63 = 26-27 MPG.
Cruise set at 65\66 = 25-26 MPG.

Again, A/C = off, Cruise = on, and enough DIC display cycles to verify reasonable accuracy. . .
These are all I had time for, before traffic intruded.

That’s all I know.
I will add, as I have posted here before, that I certainly did NOT select the G8 GT expecting the same MPG as my Corvette. I did expect to meet or exceed the EPA number, under conditions like these.
And, with premium gasoline currently near $2.00 / gallon, an additional 2 or 3 MPG would only save me something like $5.00/week – driving 15,000 miles per year.
Not enough to trouble me much . . .
- Ray
Still hoping for an improvement as break-in proceeds . . .

Is your Corvette an automatic or manual, Ray?

Also, can you look in your owner's and read up on the cylinder deactivation? I have a feeling that the mileage difference you're seeing is related to this deactivation or lack thereof. I know on other vehicles with similar technology the cylinder deactivation only functions at speeds up to 65 mph. Is the G8 the same?

"Too crappy outside to be driving"

What?!?! You may see a slippery road and an aggressive V8 RWD sedan.

I see a good time for mischievous deeds.

"Is your Corvette an automatic or manual, Ray?

Also, can you look in your owner's and read up on the cylinder deactivation? I have a feeling that the mileage difference you're seeing is related to this deactivation or lack thereof. I know on other vehicles with similar technology the cylinder deactivation only functions at speeds up to 65 mph. Is the G8 the same?" - orangutan

My Corvette was an automatic - same designation as the G8 GT's = 6L80.

The owner's manual has not a single word about AFM \ DoD \ cylinder de-activation.

In the 'real world', 65 appears to be where AFM is no longer likely to engage in the G8 GT...