SHELBY GT 350 Member
Joined: Oct 2010
Location: Desert Southwest
I hope it's okay if I chime in here, being a V6 owner. I experienced a loss of 2-3 MPG avg going from 2.73's to 3.55's.
Considering the fuel economy I'm reading about from 5.0 owners, and based upon the jump I made, I'd say you would likely lose about 1 MPG moving up from 3.31's to 3.55's.
I agree with those recommending the 3.73 upgrade. Expect minimal loss, if you can keep your foot out of it once it's done! :kooky: