Car 1:
2002 Saturn SL1 (4 cyl)
Bone stock
29 MPG (Mixed driving)
87 Octane
Car 2:
1986 Pontiac 6000 (2.8 6 cyl)
Bone Stock
26 MPG (Mixed driving)
87 Octane
Car 3:
1988 IROC-Z (5.7L)
Many bolt ons. No internal mods yet.
19 MPG (With just around town cruising. No WOT runs)
93 Octane.
1991 Toyota Tercel
Stock 1.5L 4 cylinder
168K
32 city/38 highway
2003 GTI 1.8T
60K 5 SPD
Stock except S.R. Intake
91/93 OCT + the occasional Acetone (works for me)
27-29 no acetone / 29-33 w/ acetone.
2001 Jetta GLS
265,000 miles
Bone Stock
40 city
45-47 highway
40 cetane
2005 Mitsu EVO
28,000
Modded (cams, tune, injectors, pump, intake)
18mpg city
25mpg highway
93 octane
Edit: With all these threads lately I feel like im on the TDICLUB.com page… “have you tryed this to get better fuel eco what about this” “what do you get?”… darn it just put it in and drive… nothing we do about it will make it change 
2001 Grand Prix Gt
3.8L V6 Stock
about 92k
17-19 All City driving and about 260miles per tank
65$ to fill on 87
1999 Chevy Malibu
3.1 V6
107k miles
20.96 mpg on last fill up
2006 ZX-10R
998cc
13k miles
32.98 mpg
1999 VW New Beetle TDI
147,xxx miles
Upsolute Chip, PP520 nozzles
40 Cetane
All time high 52.197MPG
Normal mixed driving 45-47MPG
All time worst in the dead of winter driving aggressively 38MPG
1991 Audi 80 Quattro
162,xxx miles
Stebro Cat-back exhaust
87 Octane
All time high 28MPG
Normal mixed driving 25MPG
Dead of winter drifitng and leaking gas 19MPG
1990 Mazda Miata
121,xxx miles
Stock
87 Octane
All time high 31.5MPG (last fillup!)
Normal 30MPG
With a day of auto-xing 27MPG
1997 Polaris Indy XLT limited
7xx Miles (yeah that’s right, it’s like new!)
89 octane
8-10MPG (eek)
2002 BMW M3
70K
Stock
29 highway, 18 driving it lol
93 Octane
forgot my other truck
1967 F-100
352Ci, 3 speed trans
2-3 k on motor
93 otc
cost me $1 a mile
1996 honda civic dx hatchback
stock ex swap with an intake
30-32 mpg city all day long even with the ac on
1994 Volvo 940
2.3L n/a 4cyl stock
196k miles
22-25MPG,highway trips are closer to 30mpg.
1997 Regal GS
87 octane
135k
23 city/30 Hwy
My daily driver:
1993 Toyota MR2
110K Miles
25-30 MPG
2007 Xterra 6MT
10k Miles
87 Octane
16 City/20Hwy/15.8 towing 6x12 enclosed trailer full of kitchen cabinets 
2004 F150
5.4 45000 miles
12-13 mpg around town
17 on the highway
2002 C5 Vette
LS1 auto 19000 miles
24.9 mpg cruising
2008 MINI Cooper
4cy auto paddle shift 1400 miles
34 mpg around town
38 on highway
All are stock motors, and all are manual transmissions.
1984 BMW 318i (M10 4cyl)
80k
36mpg, mixed driving. 89 Octane.
1991 BMW 318i (M42 4cyl)
164k
~20mpg because it’s leaking gas at the moment haha. 89 Octane.
1995 Toyota Paseo (1.5l 4cyl Tercel engine)
133k
42mpg, mostly highway. 87 Octane.
1999 Nissan Maxima (VQ30 6cyl)
183k
27mpg, mixed driving. 93 Octane.
1994 civic ex
lightly modded w/ bolt ons
City: 29(occasional rip lol)
Highway:36(cruise at 65)
87 octane@ 4.20 a gallon 
1991 Beretta GTZ
170k
Intake, exhaust, re-geared trans, 18’s are killing the mpg
27-34mpg average
1999 Honda Civic DX Hatch
stock, 84K miles
best of 42 mpg highway
usual average; mostly highway 37ish
87 octane