Pontiac G8 GT Real-World mileage

quietpro

I noticed we didn't have a real-world MPG discussion and wanted to see what others were getting.

I'm still breaking-in my G8 GT but just finished a trip. With just under 2000 miles on it, I managed 26.5 mpg on my last tank with an average speed of 67.5 mph. This was over some hilly terrain in Southern VA, NC, and SC (I-77/I-26).

That's a little slower than I normally drive (break-in) but I'm still very impressed. I'm eager to see if the mileage improves after the engine is fully broken-in.

quietpro

On the subject of fuel economy, does anyone know of a way to positively determine when the engine is operating in 4-cylinder mode? My Impala SS had an option on the DIC that would show the status but with the G8, the only way I have discovered so far is a vibration/sound that I believe indicates the 4-cylinder mode. On the Impala, this was on the same screen as instantaneous fuel economy (which the G8 does not have).

If anyone has any information, please share.

whoosierdaddy

With 6000 miles on my V6 G8, it is getting about 20 mpg for my typical mix of city/suburban/brief-hwy driving, checked by doing the math at the pump. On one long highway trip with cruise-control at 78 mph with a/c, I got 27.3 mpg. I originally thought the DIC would be self-serving rather than accurate, but I have found that it is in the ballpark so it can be trusted for an approximation of MPG. I drive mostly conservative but sometimes git on it. I leave it in SportShift most of the time because the car is more responsive that way.

quietpro

Thanks for the input. I agree with the DIC/trip computer assessment. My last car ('06 Impala SS) and my G8 GT are very accurate with the G8 being slightly better. My manual calculations are always within 1 mpg of the computer and I think the error may be mine/gas pumps since I usually push my purchase up to the nearest dollar. I've done the math and that extra 1/2 gallon or so can skew the numbers a good bit.

On a side note, I was told recently that topping off the tank, even when just enough to even out the sale price, is detrimental to the carbon can (I forget the name for it). I was told that there is an overflow tube in the fill neck or top of the tank that runs to the can and overfilling sends gas to the can and shortens its life. I was under the impression from past reading that the flow from the can is to the gas tank (that it collects unburned fuel and returns it). Anyone know the real story and have a reference to back it up?

whoosierdaddy

I have read that too, a couple of places, and now just stop when it clicks. I had been OCD about hitting an even-dollar number for so many years...one of those habits.

Also, my 3.6L V6 just seems to run a little smoother on 89 octane than 87.

quietpro

http://www.epa.gov/donttopoff/

Don't know why I didn't just look sooner. The only reason on the site that applies to me is that if you overfill, once the gas heats up, it may evaporate into the vapor collection system. The education program on this is terrible. I always figured the only problem was having gas run over the top. Well, I'll try to resist the urge to even out the dollar and see if the trip computer matches my future driving.

whoosierdaddy

My last tankful, evenly-mixed city/suburban/hwy driving, came in at 13.8 on the DIC (V6); about the same doing the math at the gas pump. I thought MPG would get better with time, not worse. Normal driving, not especially agressive, but all with A/C in the recent hot weather. Love the car so I'm OK with it unless gas goes back up to $4+.

rayainsw

"On the subject of fuel economy, does anyone know of a way to positively determine when the engine is operating in 4-cylinder mode? "

Hold down the enter key while cranking.
You'll enter engineering mode where you can see several different displays - including 4\8 cyl modes...
- Ray
2009 G8 GT @ 11,000+ miles...

whoosierdaddy

It was the A/C. Similar 90 mi drive without A/C yielded 10 mpg better. I would expect a 3 or 4 mpg hit for A/C, but 10 is excessive so will make another trip to the dealer.

rayainsw

"It was the A/C. Similar 90 mi drive without A/C yielded 10 mpg better. I would expect a 3 or 4 mpg hit for A/C, but 10 is excessive so will make another trip to the dealer. "

I would actually expect no more than 1.0 or 1.5 MPG difference.
Good luck!
- Ray
Ran a few experiments, with & without A/C, early on....

quietpro

Was the 90 miles made in one trip both times? Are you using the remote start? I can't think of any way that the A/C could drag that hard on the car and you not know it immediately (excessive drag). I'm wondering if there is some other factor involved. Any chance you used E85? It's definitely worth looking into with the dealer unless it's something that can be easily explained. Crazy! :sick:

whoosierdaddy

One trip on 2 different days with starts & stops, not identical trips but similar. No remote start, not E85 (89 octane w/10% ethanol) no other variables other than A/C or no A/C. MPG checked at the pump doing the math. I check MPG regularly and it varies from a low of 13.8 to a best of 27 on a hwy trip about 68 mph no a/c.

quietpro

Hmmm...not a likely explanation but sometimes gas stations' pumps aren't accurate. Any chance of that? It's worth checking out with the service department but I'm guessing they'll be stumped too. Then again, they may find an anaconda stuffed in your intake.

whoosierdaddy

MPG has not been that poor since. Perhaps it was a stackup of minor errors & variables. I drive a lot, and have been getting 19-23 in mixed city/suburban/hwy lately. I love the car, but it will be expensive to drive should gas return to $4.

BTW - I never trusted in-car mpg computers, I do the math at the pump. But the DIC in this G8 has been very accurate.