Thanks to an all-new lineup of engines in the 2014 Silverado, these trucks are expected to post significantly better mileage, across the line, compared to 2013.
V-8 models get EPA ratings of 16 mpg city, 23 highway with rear-wheel drive, or 16/22 with 4WD. Although if you step up to the towing-oriented 6.2-liter V-8, your mileage drops to 14 mpg city, 20 highway if you get 4WD (or 15/21 otherwise).
V-6 models earn 18 mpg city and 24 on the highway with rear-wheel drive; add 4WD and it drops to 17/22 mpg.
It's likely that these numbers will be quite reproducible in real-world driving as well. On a 42-mile loop of hilly two-lane country roads, we averaged 23 mpg in a V-6—when driven normally, and a bit briskly.