The new models use 5.3L V-8 engines and weight reduction in concert with improved aerodynamics to increase fuel economy by over 5%. Each model will get a 3.08:1 rear axle ratio along with a 6-speed transmission. The 6-speeds deep 1st gear ratio will still provide for plenty of acceleration, it's comparable to a 4-speed auto with a 3.73:1 ratio. The XFE option is available on 2wd crew cab models only.
According to GM's website, the improvements came from several small changes that are carried over from the hybrid version of the vehicles, including:
Soft tonneau cover (improves aerodynamics)
Extended front lower air dam (improves aerodynamics)
Lowered suspension and revised chassis tuning (improve aerodynamics)
Aluminum lower control arms (reduce mass)
Aluminum spare wheel (reduces mass)
Seventeen-inch aluminum wheels (reduce mass) Tahoes & Yukons get 18s
Automatic locking rear differential
Low rolling resistance tires
The result is a 15/21 rating, which will help Chevrolet when Ford and Dodge come out with their '09 EPA ratings, which will both increase.