gas milage

i got a crate engine is the 350/290hp, i installed the holley commander 950 700 cfm tbi unit has the wide band o2 upgrade and computer control timing, timing is set at 12deg btc and a total of 34deg also vacumm guage reads 14inches @ idle and closed loop read 14.7o2 reading it has a 350 tranmission with a 1500 stall converter all of this is installed in a 1986 cutlas supreme 2 door my best milage was 8 mpg how can i make it better any suggestions please.  

Overdrive.  

14 inches of vacuum at idle sounds awfully low for that motor too.  From what I remember, the cam isn't that hot in that motor...so you may have a vacuum leak somewhere.  
68 Olds Cutlass ~ 350 Rocket
85 Delta 88 ~ 425 Rocket
02 Silverado 4X4 ~ 5.3L

Sounds like a mis-calculation to me.  Are you saying with all that you only got about 100 miles out of the tank?  Someone is siphoning your gas. With 14inHg at idle and all the efi tuned up you should be getting bare minimum 12mpg.  My Centurion doesn't go below 12mpg on normal city driving with an old carb a 455 and 500 to 1000 more pounds of weight.  
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The Silver Buick- '77 Skylark coupe w/ a Fuel Injected Buick 455&TKO-600, '72 Centurion Conv't - 455w/TH400, '67 T-bird 4Dr (suicide) w/428&C6. Needing to replace a '69 Firebird 400.

http://www.bangshift.com/forum/index.php?topic=6189.0

siphonig? that is funny cause i tought the same thing, but my calculations have been going on for six months, and the car is in a secure place, yes i have looked for a vacumm leak none found, i have sprayed the intake and idle stays the same, could it be the intake manifold i use the air gap from eldrebrok? and yes i only get 100 miles from the tank.  

Have you checked your plugs?  They may give you some insight to whether or not you are running rich.

I wouldn't imagine the intake manifold is the problem...not THAT much of a problem anyway.  
68 Olds Cutlass ~ 350 Rocket
85 Delta 88 ~ 425 Rocket
02 Silverado 4X4 ~ 5.3L

did you swap the rear gears or the tranny? the speedo could be reading wrong. at one time i was pulling 24/32mpg on paper, but only about 11/15 at the pump  
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It may not be the fastest or prettiest, but i can pretend

squarebackjalopy:
did you swap the rear gears or the tranny? the speedo could be reading wrong. at one time i was pulling 24/32mpg on paper, but only about 11/15 at the pump

Brings up a great point.  You did swap in a transmission...even if you didn't touch the rear end, the transmission speedometer gears could be setup for something entirely different.  
68 Olds Cutlass ~ 350 Rocket
85 Delta 88 ~ 425 Rocket
02 Silverado 4X4 ~ 5.3L