First off, I not very technical when it comes to carbs so bare with me. Not sure what info you need so ask me if there's something I missed. I had my motor rebuilt, 30 over with mild cam, lifters, pistons, rods, ported & polished heads, new crank ect.... Before the rebuild, I had an Edelbrock 670, performance was good & gas mileage not bad as long as I stayed out of it. Now I have a Holley Street Avenger 770 & my mileage sucks bad. I'd like to stay with Holley but not sure if I should change it & to what CFM. Any idea's would be great, hopefully from your personal experience, not what you've heard or guessing at what I should do please.

P.S. I drive my car slow & easy only going to car shows & cruises. Didn't know if that might help. Thanks

 

650cfm should be plenty for the motor, Are you running a vac advance on the distributor? and what mpg are you getting now?

 

I've been reading about the Street Avengers on various forums. Sounds like a lot of people have issues with gas mileage and power, among other things. My advice would be to type "Holley Street Avenger 770" into Google or something and read some of the comments. Might be worth putting your old carb back on and see how it performs.

 

What motor is in it 350 or 455 ? Those engines weren't gas misers by any means when they were new so with a cam and ported heads your making more HP which equals more gas .On my 350 run a Quadrajet which has small primaries for tooling around but big secondaries when you want to get on it .It's a good combination for a street car.
I don't worry too much about gas mileage as I don't take it on long trips , I've got my Mazda 6 for that.

 

It could be the new cam in your rebuilt engine. I used to get 12 mpg, then I swapped cams (nothing else) and now I get 8 mpg. Higher lift and longer duration allow more air/fuel into the engine.