I noticed that I get about 290 miles to the tank (310 corrected for Nittos) in the winter, but can squeeze 310 (332) out in the summer. I live in northern VA, so the differences are noticable, but certainly not what I would call extreme.

Colder, denser air = more oxygen content per given volumer = more aggresive combustion?

My Yakima rack with full-sized spare costs me about 20 miles per tank. Interestingly enough, my rack WITHOUT the tire has the same penalty.

Ron