OK so im planning on stroking my lt1 to 383 and to put a cc 306 in it...my question is what type of gas mileage am i going to get with that cam...also is it a lot more mpg than if i put a 503 in it?

 

The bigger your displacement, the more air you move through the combustion chamber. When you move more air you need to keep a good air/fuel ratio, so yes your gas mileage efficiency is directly related to displacement. now you can lower its fuel needs by choking it with small heads and a small intake, but its not recommended.

My 408ci in the LS has home-ported stock heads and an LS1 intake so I'm not dumping as much air or fuel into it as I could, and I actually get great mileage from mine. I am usually around 18 in town and 23-28 on highway which is just a few mpg lower than when I was stock. (of course thats an LS example not LT)

 

I still get around 25mpg highway with my 6 speed 383ci ws6. 3.73 rear gears. Great car for cruising.

 

btw- neither of the two cams you mentioned are very good choices for a 383.

 

383 and gas mileage shouldn't be in the same sentence.