3.8l supercharged gas miliage?

[forrester1990]

Hi, my girlfriend owns a 04 gtp with about 132,000 miles, and we get terrible gas miliage(well what i think is terrible).
I would say it gets about 20-22 on the highway. We have tried all the thing that we thought would help but none so far.
We have changed the fuel filter, plugs and wires, oil and filter, air filter.
Just changed the thermostat yesterday.

What have some of you done to help miliage or could something be wrong?
I have noticed it doesn't go into boost as quickly as it used to, When i changed the thermostat i checked all the vacume lines and didnt notice any cracked.

Thanks

[dsmuts]

Pull the throttle body off and clean it.

[redlinepontiac]

a tune was biggest help on mileage

[Fivefingerdeathpunch]

any ses lights on?

what speeds are you going at the freeway aswell?

[redlinepontiac]

65 is better then 75

[Fivefingerdeathpunch]

I liked that twice.

[KeithGTP03]

How are you calculating mileage? By the DIC? Or are you calculating by driving then refilling?

[forrester1990]

Just going by dic. we drove to evansville last tuesday( about 90 one way) and it took $40 in gas to get there and back. Mostly going between 55-65 mph. Havent took throttle body off yet, but may have to, to see if it helps. Alos no ses lights, Just one a week or so ago for Coolant temp below thermostat regulating temp, Thats why i changed the thermostat.

[ctracer]

you changed the thermostat to stock 195 right?

[buickman104]

With a 180 t-stat Gen 5 and 18 inch rims I still get between 31-34 MPG highway. New York City driving I get 14MPG. All about a half decent tune and making sure the car is mechanically sound.

[forrester1990]

yea, stock factory 195. Car runs good, shakes a little but it has done that ever since we got it, Even after we changed the plugs and wires.

[DruMor]

On a long trip, if I run 60-65 according the dic my 2004 Comp-G (101000mi) got up to about 24.5 and when I hit the 4 lane and sped up to 70-75 it dropped back to about 22MPG. On my normal work commute, 10 of 13 miles is a straight flat 60mph drive and the DIC says about 21.5 if I'm nice to it, and calculating it at the gas pump is usually within a couple tenths of a MPG of the same.....if I drive it sensenably. When I do what I did today and the wife and I go on a 100 mile road trip around the county on windy hilly 2 lane, the DIC may say 21 but the actual is closer to 15-16MPG. But that was foot to the floor, foot on the brake, foot back to the floor TapShifting thru turns, screwing off and having a good time. I changed the plugs out at 100,000mi and didnt see any change in MPG.

On the same streach of road for my daily commute, my 2002 GT would get about 24MPG calculating at the pump and according to the DIC about 26MPG.

[dsmuts]

What kind of plugs did you put in?

[forrester1990]

If i remeber right we put the iridium plugs back in.

[SgtMarshal]

my '99 gets 20 in and around the city and 28 on the highway but it has a full intake and full exhaust.