If I have a 235/70-15 tire, what's its diameter in inches? How do you figure that out?

I guess the best way is to mesure it, but I think you can calculate it like this :

The tire is 235mm wide. 235/25.6=9,18 inches
The hight of the tire is 70% of 235mm. (235*70)/100=164.5mm 164.5/25.6=6.43 inches
The diameter og the wheel should be (6.43*2)+15=27.86 inches.

I use this calculator:
Miata Garage - Tire Size Calculator (http://www.miata.net/garage/tirecalc.html)

or the older version:
Miata Garage - Old Tire Size Calculator (http://www.miata.net/garage/tirecalcold.html)

I think that the new calc must have Sun Java installed to work, where the old one does not... Something like that. Either of those will tell you more than what you requested.

*EDIT* - posted at the same time as Norman. Yes, the formulas he gave will work. I just find it easier to plug it in to the calculators above for more information.