I was wondering this too. I just put on 33/12.5s and have lost over 3 mpg from the stock 32s. I'm thinking they must be under inflated because I can't see the few extra pounds per wheel cutting more than 15% of my fuel efficiency. Haven't done the chalk test yet though...
Normal people believe that if it ain't broke, don't fix it. Engineers believe that if it ain't broke, it doesn't have enough features yet.
- Scott Adams