I usually get about 23 city/ 32 hwy. One time, I was on the freeway late at night, with all green lights and little traffic, I got 36 mpg. I was driving about 60 MPH. If the gas didn't have 10% ethanol in it, I probably would get 35 MPG all the time on the freeway. Not bad for a good size car..

I have the factory 3.4 and it hasn't had any mods done to it (yet)..

Dual exhaust would be nice- that might be soon, considering it's the only thing on the car that's rusting..

 

After installing a zzp downpipe and SLP powerflo exhaust system I'm getting around 31-32 highway and 18-20 driving around in town. Should be doing the SD headers and CAI within the next few weeks so I'll have to give an update on mpg after that.

 

I went to North Carolina for Thanksgiving and got 33mpg (calculated by hand). Drove part of it on the backroads (meaning I had to slow down for small towns, etc) and then the rest on the interstate. I averaged around 77 or so on the interstate but for about a 40-50 mile stretch east of Knoxville traffic was flowing around 90mph so I stayed with the flow. Bone stock L36 (N/A 3800). Haven't even done an intake or cat-back.

04 Dale Jr (L67) I used to have got right at 30 on the highway and a 96 Camaro I used to have (RWD L36 and 5 speed) I got 35mpg hwy with a CAI and catback. I probably would have been able to eek more mileage out of it if the cat hadn't crapped out on me and if I had let it breathe a little better.

 

2003 ss 3.8 n/a heavily modded.

City/hwy combined best was 31 mpg's (average speed 64 km/h)

Hwy mostly (95%) best was 37.48 mpg's (@130-140 km/h)

Regularly achieving 34-35 mpg's

Overhead display is accurate within .5 mpg's

Always work it out with pen/paper/calculator.

Beating on it like normal 21 mpg's

If I was driving all hwy and took it easy I could hit 40 I'm sure.

 

22-23.5 mpg everyday driving. I live in a rural area, it's not really city, but not really highway either. I never got better than 24 mpg, highway trips included (70-75 mph). I don't know why the mileage is subpar compared to other 3800's, I don't drive like an idiot. I even changed the fuel filter after someone suggested it. I think I may try spark plugs next.
My dad's '02 LS with the 3400 gets 25-26 mpg all day long and occasionally I've hit 27 mpg on the highway with it.
I always calculate at each fill-up, the trip computer is 2.5-3 mpg too optimistic. lol