I don't know that it is an issue about being careful about MPG, but more like what is attractive about each car. I really hate to make these sweeping statements, but I think NAH buyers tend to be more interested in performance (the Altima has the more aggressive styling, suspension, and power) and will want to take advantage of that, consciously or subconsciously, which will result in lower MPG. TCH buyers are a slower-paced lot, interested in comfort and a easy-going ride. Much more likely to drive in an economical way.

(I've probably insulted half each of the NAH and TCH owners here!)