Happy and proud to say, that when I actually look at MPG at is about 10 to 12.
I did not buy this car for MPG, that's what Toyota's are for.

 

I'm taking it you have a lead foot?
Of course we didn't buy these cars for MPG, but that is awfully low.. but then the 2011 v6 is looking at about 30 MPG and sure beats a Toyota for fun level.

 

A CAI & tune may improve mileage by about 1 MPG IF you're easy on the gas. Except a CAI & tune has the nasty habit of making your right foot heavier...

 

GT with manual transmission and fuel economy calculated by hand

nearly stock - 20 city 28 highway (3.31 gears)
3.73 gears - 18 city 21 highway
jba shorty headers - 18 city 25 highway

i also have a cai, tune, and the stuff listed below, but it seems like the gears and headers were the only modifications that significantly affected mpg. i calculate mpg by hand every time i fill up and the computer reads 2-3 mpg higher than actual when driving on the highway and 1-2 mpg lower when driving in the city. i dont think its ever been completely correct in ~100 fillups

 

MPG calculated by hand at every fill up, 22-23 total driving.
I mostly run highway 70-75 mph 75%, and city 25%. 5sp manual and 3:31
gears, w/ CAI and tune. I have seen as high as 26 while on a trip, all highway.

 

I get on average 15 mpg with a mix of city and highway driving. I have a GT with a CAI and tune. I would say since I'm constantly driving fast and hard that is why I get such poor mileage. But I didn't buy this car for gas mileage. I bought it for fun and a thrill.

 

I'm typically looking at 19 or 18 combined. Hand calc'd. Never have seen over 21.
3.55, cai, tune, o/r h and most of the time a foot thats not so light. I usually be nice to it but there are times that im in the mood for very spirited driving. On the hwy honestly 80mph seems to be her sweet spot. I've tried anywhere from 60-85.

 

I would think an aluminum DS would benefit mpg, yes? Thats like a sold ~25lbs. removed from the drivetrain?

 

No way. These engines begin to gulp fuel once they run over 2000 RPM. With 3.55's, you're turning over 2400-2500 RPM or so, depending on your tire OD. The sweet spot is around 1600-1800 RPM. THAT'S why you never see 21 MPG. LOL!

 

On trips over 100 miles on the interstate driving around 60 mph I get 30-32 mpg. If I drive 70 mph I get around 25-27 mpg. So, driving habits play a big role. In my local area I average around 25-27 mpg. I have a 2008 GT/CS.