1st question....
Di you RESET your MPG on your DIC?
If not- then youre getting average since last time it was reset.
Fill up tank- and reset the parameters on the DIC.

We have an 07 3LT with the 3.9. In city/hwy combined we get 19-22 MPG. depending how much we do of each.
On ALL highway trips up the coast to Santa Barbara I have managed 31 MPG- this was achieved by doing 55-60 and using cruise control.....

as a Comparison----
Just last week- we took a trip from Los Angeles to San Jose to San Francisco in my new 2010 Chevy Traverse (this has the 3.6 Direct enjection engine). 4 adults- some luggage and staying between 67-70 mph.
Best mileage seen was 23 MPG. worst-21 mpg (up the Grapevine).
This is based on DIC and HAND calculations. I have found that the DIC in the Traverse is pretty accurate.

 

I would not expect a super good millage per gallon since the 3.9 liter is exactly the same engine as the uplander 3.9 with the same axle ratio of 3.29. I have a uplander and I can achieve around 7.6 liters on flat highways and no wind situation driving at 90 kilometers. If I go faster and windy I will get around 9 liters per 100 kilometers. It all depend how you drive and the situation you are in.

 

I stand corrected 07SSport. I've also had good luck with Lucas Fuel treatment in-town. Anyone else??

 

woah....wait a minute. how does the computer automatically adjust to differing levels of fuel combustion? the puter doesnt know if you're putting in 89 or 93 octane gas, or if you're using E85. it has nothing to do with the compression ratio, its all about the timing of the engine, when the plug fires to ignite the fuel. the timing on my 3.4 impala is 22* over stock timing. therefore i have to run premium(91 octane or better) or i wll get severe knock because the gas isnt igniting at the right time. with that said i can easily get 35 mpgs doing totally highway driving with my 3.4. the 3.9 is a 60* v6 just like my 3.4. the 3.9 isnt all that much bigger than my 3.4 either.
if the knock sensors pick up knock it will retard the timing, in other words back off timing. this will result in less HP and milage. 07SSport, thats the reason you get worse milage when you run less than 91 octane in your LS4. the sensors are picking up the knock from the gas detonating before its supposed to. your LS4 will get worse milage with anything less than 91 octane because the timing of the LS4 is tuned specifically for 91 or better octane gas (thats why in your manual it states to use premium fuel only).
the octane rating of gas is just a measurement of how prone it is to pre-detonation. 87 octane gas is far more prone to pre-detonation than 93 octane is.
mlrdking, how many people and how much luggage did you have in the car on the trip? weight will also effect you mpgs. also, dont rely on the trip computer to calculate you mpgs. they have been known to be way off, especially on the 2000-2005 models. i dont expect that to be any different in the 06+ models.

 

THis is what I meant when I stated the timing adjusts to the different fuel levels... When an lower grade gasoline is used the timing will adjust....

I find it hard to believe that the 3.4L Impala requires 91+ octane. My father has driven one since 2004 with regular unleaded and has always gotten over 30 MPG. But hey its your money, not mine. If you feel more comfortable putting 91 in it, its not hurting anyone.

 

the puter will adjust when it starts picking up knock from pre detonation, yes. but if an engine is tuned to use a higher grade gasoline, like your ls4, it can only back off the timing so much, because the timing is advanced so far. thats why you will get knock and worse mpgs/hp with a lower grade gasoline. a stock pcm, 2000-05, (ecm for the 06+) doesnt have the timing advanced as far as mine. thats why you're able to use regular 87 octane with a stock tune.
i have a tuned pcm, with the timing advanced. if i put in anything less than 91 octane i will get severe knock and missfires. so yes my 3.4 requires 91 or better octane. i also have a 180* thermostat and the fan settings are set for the 180* thermostat so my engine runs cooler than a stock 3.4. that way i dont get pre detonation from the advanced timing.

 

Now I am on the same page. Did not realize your PCM was tuned.


I still think that on a stock 3.9L, using a grade of gasoline higher than reccommended will be detrimental to gas mileage, and at the very least will not be any BETTER. In which case it would be nothing more than a waste of money.

 

my 2k 3.4 has never gotten better than 18 in town and 24 highway

 

My Impala developed a coolant leak last week. I took it to a local Chevy dealer and they determined the head gasket was leaking. With 41000 miles I didn't there was any waranty left. But the service guy said there was a TSB on the headgaskets for 3.9L. So they fixed it for $0.
It may just be psychological, but I think it runs better. Maybe the mileage will improve.
I have to take it back because the Service Theft Deterant System message stays on and the icon is lit up too.

 

Thats awesome they fixed it...I thought TSBs did not require the dealer to fix anything, that they were just service bulletins (kind of FYIs).... Very nice.