Okay, I know the question has been asked a couple dozen times (literally, just search ), and I'd like to form a post with all of the information right here - all in one place. So what I'd like people to do is answer the following questions. I wanted to go through old posts to re-construct MPG claims, but with CAI'* becoming more common, I couldn't keep track of who got what when. Soooo, if you would answer in the following format, I would appreciate it. Once the replies start coming in, I'll edit this first post to reflect the people who have responded. If I'm feeling really motivated, I'll throw together a table to put on the web as a constant reference...
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year: 1996
model: lesabre (bonnies, just put se/sse/ssei/sle)
engine: L36
CAI?: Yes
Other Mods?:No

Best Observed HW MPG: 32 MPG
Average Observed MPG: 27 MPG

Best Observed City MPG: 28 MPG
Average Observed City MPG 20 MPG

Year: 1997
Model: Grand Prix GTP
Engine: Series II L67
CAI?: Yes
Other Mods?: 3" downpipe, High-flow Cat, U-Bend delete, Ported Exhaust Manifolds, 3.4" pulley, PCM, 180 t-stat, Autolite 104 plugs

Best Observed HW MPG: 34 MPG
Average Observed MPG: 29-30 MPG

Best Observed City MPG: 27 MPG
Average Observed City MPG 24-25 MPG

Final Drive Ratio: 2.93

I'm doing this because I have the L67 engine, and the same results can be seen on a Bonneville.

year: 1993
model: Bonneville SLE
engine: L27
CAI?: Yes
Other Mods?: N10 high flow muffler

Best Observed HW MPG: 38 MPG
Average Observed MPG: 25 MPG

Best Observed City MPG: 15 MPG
Average Observed City MPG 13 MPG
(should be noted that I have alot of starts and stops in the city)

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year: 1992
model: SSEi
engine: Series I L67
CAI?: No
Other Mods?:No

Best Observed HW MPG: 24MPG
Average Observed MPG: 23 MPG

Best Observed City MPG: 22MPG
Average Observed City MPG 19 MPG

Mileage 129,300 mi
O2 sensor: ???? mi

year: 1994
model: Eighty Eight
engine: L27
CAI?: No
Other Mods?: Magnaflow High Flow Cat

Best Observed HW MPG: 33 MPG
Average Observed MPG: 29 MPG
(summer)

Best Observed City MPG: 26 MPG
Average Observed City MPG 22 MPG

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year: 1989
model: LE
engine: LN3
CAI?: No
Other Mods?:No

Best Observed HW MPG: 30 MPG
Average Observed MPG: 26 MPG

Best Observed City MPG: 25 MPG
Average Observed City MPG 19 MPG
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year: 1987
model: Ninety Eight
engine: LG3
CAI?: No
Other Mods?:Generic High Flow Cat

Best Observed HW MPG: 28 MPG
Average Observed MPG: 27 MPG

Best Observed City MPG: 22 MPG
Average Observed City MPG 18 MPG

97 SE all original...no mods...best hwy 34 mpg...best combined ......28 mpg.....

Year: 1993
Model: Bonneville SLE
Engine: L27
CAI?: Yes
Other Mods?: umm it was a really crappy figure for my..

Best Observed HW MPG: 40
Average Observed MPG: 27

Best Observed City MPG: 26
Average Observed City MPG 20-22

You forgot one VERY important factor..
Gear Ratio : 3.06

my bad. forgot that not everyone here has 3.06...

Year: 1998
Model: SLE
Engine: L36
CAI: Yes(FWI)
Other Mods: 1.9 Rockers w/ other valvetrain componets, PCM, PEM'*, hi flow cat/resonator/muffler

Best observed HWY MPG: 41
Average HWY MPG: 25

Best observed city MPG: 25
Average city MPG: 23
(I have a heavy foot )

year: 1999
model: Bonnie SE
engine: L36
CAI?: No
Other Mods?:None

Best Observed HW MPG: 30 MPG
Average Observed MPG: 24 MPG

Best Observed City MPG: 25 MPG
Average Observed City MPG 22 MPG

Original oxygen sensor. Plugs, wires 35K old. Total mileage 100K