There's nothing stopping you from looking up other Altimas in the Ecomodder garage pages, but since you asked:
2005 Nissan Altima SE Gas Mileage (Slowest. Altima. Ever.) - EcoModder.com
His best results are getting into hybrid territory. I checked; his is also a V6. His techniques and the two modest modifications have been thoroughly tested by many others in these pages.