The average girth is below 5", period. If you look at the Lifestyles survey (the most thorough survey done so far), the average is like 4.9", but the median is only 4.6, and this is the size at which half the cocks are larger and half are smaller. And I'm certain that a lot of smaller guys did not want to be measured, even when partying on spring break, and that will skew the results higher as well.
I know it seems odd, because even on here we all look down and see ourselves and think we're maybe average. But we're not. 5.5" girth here, and I wish I had more, but the proliferation of porn and the big dicks in them (plus the camera angles and all that) make us think we're smaller than we are.