...if you're after MPG, the only way to go is the 6-speed manual Eco Cruze, the 6-speed automatc LTZ isn't gonna deliver anywhere nearly the MPG.
...summary: auto = no, manual = yes, especially the eco model.
...look around, the 2012 Cruzes offer manual trans on all 'LT' models except the LTZ...and, be advised the LTZ is also the heaviest model at OVER 3300 lbs typically.