SkyHi gave you everything you need, sometimes I go to a site like tirerack.com and if you look at a list of tires, they will show the O.D. (Overall Diameter) of the tire. It's very helpful since 2 tires with the same size by different companies will come up different heights.
Also, it helps just looking at the O.D. if you are willing to got wider or more narrow on width just for the right height.
Is there any specail raeson why someone might want to go wit 31/10.5 over a 255/70? Everyone had been telling my to go with the 31s on my XJ but when I went to the store the salesman wanted to seel me 255/70-15. He said that they would give me the width of the 31 with the hight of the 235/75-15s that I'm running now. I don't need more clearance, just more width & I'd like to do it without putting in a lift.