I've noticed since Sept that my gas mileage has been getting worse. I drive down this big hill every day to work and during the summer when coasting down this hill (w/ foot off gas) I would actually pick up speed. For the last couple of months now coasting down this same hill, i actually start to slow down. Could the cold weather be having that much affect on my lube/bearings? Is something else going on here? I'm experiencing the same thing with my Tundra (although that beast never picked up speed going down hills).
Um, tire pressure shouldn't have THAT much affect on your downhill coasting. Up here in the Frozen North, it gets down to -20 (Celsius) regularly, and it's still not necessary to add air (down to -30 degrees, well that's another story). I've had the same problem in all three of my cars and it's either been: a) slightly or very seized caliper, or b) wheel bearing.
Considering costs, try the "adding air to the tires" approach first and hope for the best...