I rented one for a weekend trip to Ocean City, MD from Boston and back about 1,000 miles down, back and about locally. IIRC probably best MPG I saw might have been around 20 MPG. Almost all highway.
IIRC the one I had was original 3.7 engine. I believe the 08 to have the Pentastar 3.6 and newer transmission so MPG might be slightly better.