I would venture to say that gas mileage in a high traffic, stop and go environment would be better, as the car does not have to work as hard to get moving from a stop... I imagine the only mpg loss would be on the hwy, however...
http://www.camaro5.com/forums/showpo...4&postcount=11
Read from post 11 onward regarding fitment of the 3.55 gears in a 2ls rear.