One thing I think really changes the power is the weight of the tire also. Here is what I found when looking for tires for my Trooper. This is for BFG MT's:

31x10.50 weighs 43lbs

33x10.50 weighs 48lbs

33x12.50 weighs 64lbs

Going from a 31 to 33x10.50 is only 5lbs per tire, but to a 33x12.50 is 21lbs per tire. So for 4 33x10.5 it is only 20lbs extra of tire and for a 33x12.5 would be 84lbs extra of tire. That is alot of rotating mass, almost like having a 6wheel 31x10.50 Trooper .

Also, think about this. If you have almost bald 31x10.5's and go with 33x12.5's, I bet that would be over 100lbs different. That would seem to make a big difference.

L8r,

Kevin