Go to http://www.1010tires.com/TireSizeCalculator.asp
This will help you compute everything, even how much your speedo is going to be off.
You can get your speedo recalibrate, I think Superlift sells a controller for it, as well as an Xcal 2, hell you can even got to the stealership and they will be able to do it for you. You are going to have to pay a pretty penny though.
And think about the 31" tire size. It is 31" in circumference, hence the 31"
Rim size does not affect your speedo caliubration. It is the diameter of your tire that will do it. If your going from a 31x14 to a 31 x 15, the outside tires size is the same, but your profile (sidewall) of the tire changes.