how much of a difference in gas mileage would i notice between a stock carb'd 305 and upgrading to a holley high rise dual plane intake manifold with 1" carb spacer and edelbrock 600 performer series carb? is it worth it...?

 

or could I use my stock carb with the new intake?

 

a correctly tuned carb will only decrease your mileage but 2miles in a fuel injected car, thus is your car it can either go up or stay the same, if it goes down its not tuned right.

 

so since my car is carb'd...with an aftermarket intake and carb...and i properly tune the carb...my gas mileage will either go up or stay that same...even though the carb has a higher cfm???

 

correct if it properly tuned... higher cfm means ur only filling the engine with more air... u need to really have the right cfm for ur engine, over flowing it will just bog it down

 

Now why would you want to downgrade from the stock carb to an Edelbrock?

For that matter, why do you want to go with an aftermarket intake manifold? You weren't specific about what all you've done to the engine/car, but carb and intake are the very, very last things to replace on a factory carb'd 3rd gen, and for the street, you probably shouldn't even consider replacing the carb.

Going with a larger carb will decrease gas mileage. There is no way around that. Leaner jets won't allow the engine to run properly if it's so lean that it gets the same gas mileage.

Think about it: Why does the q-jet has those small primary venturi, and huge secondaries?

 

57kid is right. Larger carb.....more stops at the pump. My carb is correctly tuned for my motor (I think. I'll know as soon as my delivery problem is fixed.), and I went from 17 city stock to 12 mostly highway. But thats with a 200 + HP increase.