Well after reading that tire test article maybe it explains why I wasn't suddenly faster when I changed from my 700x38 to 700x28's.... maybe it's not so much of a speed inducer, especially for someone who rides few flats roads.

And Lauraelmore- You make me feel a LOT better when you say: "When I talk about achieving an average mph of 12-13, it's on very flat, very smooth trails. I'm not likely to mention that when I ride on the roads around my house, which are mostly rolling hills, my average speed is only 9.5-10 mph.(and that's if I'm really pushing it)".