19. OHIO STATE (13-1) – 97.1% (Ohio Stadium holds 104,944)
That means an average of 101,900 leaving 3,000 empty seats every game. (On average)
Part of that is the 3 lame games each year against Toledo or someone we aren't interested in.
I see another issue with how seats are sold though. Each year when individual seats go on sale, you see a ton of single seats all through the stadium. A system that prevents these "dead" seats forcing you to move your seat selection to another area to always keep at least 2 seats together would help. I've seen this done at the movie theater and other shows.
Finally, cracking down on re-sellers would really help. Scalpers buy up as many tickets as they can, then charge more money, often more than people are willing to pay.