Does anyone know the inner dimensions of the rings?
Game Manual, Part 2, Section 1.5, Game Definitions:
“Ring – One of the Scoring elements for the 2012-13 FIRST Tech Challenge game, Ring It Up! The Ring is a 4.75” (12cm) outer diameter (O.D.) and a 2.25” (5.7cm) inner diameter (I.D.) Alliance-colored plastic toroid. There are three types of Rings: Autonomous, Normal, and Weighted.”
Your answer is 2.25".
At the kickoff yesterday we were able to see and touch the field up close. The rings fit snugly on the pegs, and cannot be taken off by pulling straight back (they have to be titled to get off the end). I’m not sure if that’s helpful for you at all, but I thought I’d share it since most people haven’t gotten the chance to look at a field up close.
The FTC ring sets maybe out of stock now but we bought two sets of Fisher
Price baby “rock-a-stack” toys at Walmart for 6.95 each . The last ring in the set is the correct size. 
Thanks so much for the investigation!