I knew it was bad, didn't know it was that bad.
16.4 L/100Km
14.3 MPG
AVERAGE on the DIC That's worse than the Camaro ZL1, so something seriously wrong.

When it was newer, I remember it being at 17.9 average for a long time. I tried to get it higher by doing long trips. It was better than that after a California trip. (We drove 600 miles to see a 500 mile race)( it was a route 66, 2 week trip)

 

ouch...that is pretty bad. Have you done anything to it?

 

If gas mileage is a main concern, then you should be looking to do one or more of the below:
-smaller engine (ie v6 or boosted i4)
-smaller/lighter car
-more gears

As mentioned, that 21 figure is low for all highway, but really- city mileage is miserable. Not to mention, the car is supposed to be run on premium fuel, so you're adding even more to the cost of refueling.

It wasn't designed as a fuel economy car though. It's the biggest engine you could buy in a GM fwd car with an outdated 4 speed auto.

Sure under special ideal condition testing, you can squeeze pretty good mileage out of it (easy take offs, driving the speed limit, etc)- but then you aren't really enjoying all the power. To enjoy the power/torque of the car, you'll need to get on it, which will in turn kill gas mileage.

If you get into newer cars with DI, small displacement, a turbo, and a modern transmission, you can have a better mix of both worlds (power and fuel economy).

 

Last Year,
new plugs and wires. Then New coils.
New Oil Pressure Sensor switch.
Re-Built Transmission by Dave at TEP.
Tune by Will at Overkill. (in car)
MAP & MAF sensors.
Nitto 555 tires.
and yesterday, new temp control actuator on the driver-side.

I don't know why mileage is so bad.
I think maybe it needs a Fuel system cleaner?

 

It looks that way.You should be getting way better than that.

 

If your looking for a Monte but want good mileage and still have good power go with a s/c 3.8. I get 31-33 mpg on the highway!

 

New coils, how many miles you have on this? I have replaced 1 bad coil on a ls motor in 8years. You replaced how many? Why did you replace coils after new plugs and wires?, misfire or just because? what plugs are u running and gap.
To be honest fuel mileage on these ls4 motors are horrible at best for city driving. Highway somewhat better. mines 14.5 average city with tuning.

 

I may as well add to this thread.

On my way to MIS in Michigan the other day from Toronto, I filled up and left and drove the 4 hours straight without stopping and drove on average about 120km -125 km/ hr or 70- 75 mph. I did not have to refill until I was going home again.

I did not check my information center for the gas mileage right away upon my arrival and did some stop and go driving for about 1hr 30 minutes before we arrived at MIS. When I did check it the info center said I averaged 25 mpg. I am sure if I drove 100km an hour or 60mph I could have averaged more.

When I refilled my tank after that long drive the fuel range said I could get 620 km or 385 miles.

I just had my spark plugs changed -- I did not change my coils as it wasn't really necessary as KillerB said. My oil changes are synthetic and I do an engine clean after about every 4 or 5 oil changes. Take it for what its worth.

 

Yes, there was a misfire, after new AutoLite XL plugs and can't remember name, high performance wires. That was at 210,000km.
They offered to buy sell me a new coil for almost 100. Said it might need more than 1, they looked burnt.
I bought them online for 30 each, so 8 of them, with shipping was just under 300. That was January 2013, just before the Tranny re-build.
Still had minor, random misfires, couldn't find anything specific, but went away after MAF & MAP.
The tune made it more aggressive, so definitely hurt mileage.
I'm now at 230,000km

 

How in the world do you manage that? Best i ever got with my car was 25mpg