Thanks to TacomaWorld.com for publishing this.
It calculates the differences in tire size, which can be used to figure out what size tire to run on a bigger or smaller rim. It is also convenient because it has additional information, such as speedometer variances.
http://www.tacomaworld.com/forum/tirecalc.php
Here's an offset calculator too.
http://www.1010tires.com/Tools/Wheel-Offset-Calculator
Oh, and a backspacing/ misc. calculators
http://www.customwheeloffset.com/tir...et-calculators