Gears & Axles - Needing to find out some gear info. Bigger tire and biggger gear.

I have a buddy that has 4.88 gears with a 31" tire. I am trying to figure out what that would be equivalent to. 3.73's and 27"?

Does anybody have a link to a calculator for it? I couldn't find one on Google.

4.30s'

http://www.f-body.org/gears/