GMC rates the Terrain as 17/24 for the FWD V6, and 16/23 for the AWD V6. We beat those ratings in real world driving. Not by much, but a little.
Acadia is rated the same, but more towing, and overall larger vehicle. My wife wishes she had the Acadia rather than the Terrain just for the space when we load 2 adults, 2 kids, 2 labs, and bags for a weekend trip.... We usually end up taking my truck, or I ride the C14 while she drives her Terrain.