i am starting to shed weight on my 02 tahoe, and i want to know if i go for ls1 out of an f-body would i lose mpg's or gain mpg's? because of the lighter aluminum block, but the ls1 having a higher displacement and all i worry that it would make me lose more mpg's. The reason im worrying about this is because i daily this thing, also i plan on running a t56 also out of an f-body.

 

I don't think the cost VS gain is there at all, Losing 80pounds off a 5500lb truck isn't going to net a MPG gain. Not to mention you better plan on building up that T56 to handle the extra bulk, They let go behind bolt on Fbody's that weigh 3500lbs. Adding 2000lbs to the equation and expecting any type of spirited driving will not end well for the transmission.

 

driving habits will determine your MPGs. If you "get on it" a lot, then the MPGs will suffer. My suburban gets 8 MPG with my "spirited" driving style.