the bedrock of the NFL is the college/university system. It's the main conduit into professional sport. They have a loyal and huge fan base, 100,000 every game in some cases. The "tribal" mentality that is prevalent in European football is barely evident, tail-gate meets often are home and away fans, where the "craic" is more important than loyalty to the teams. Each team has pockets of fanatics but generally a small but noisy percentage. The college teams have a following more akin to our set-up, but really , still a world away from the atmosphere that pervades football in the UK and Europe. In summary, any attempt to compare will have so many caveats as to be meaningless imo.