First Trip out with Dodge Durango, Great Surprise on MPG

We were pulling out TM with a 6 cylinder Isuzu and had trouble on a lot of mountains. Purchased an 04 Durango with Hemi and got 15 mpg on the first day. 15 mpg was really the best I could expect on the Isuzu after pulling it for over 30,000 miles. A few times we got more with the wind at our back. 15 mpg was pulling in Florida on flat road with a 10 mph cross wind. Isuzu was probably underpowered and working too hard. Great surprise. I expected less. I thought anything over 12 mpg and I would be a happy camper with the NTU Durango.