^ I love that calculator.

Ok. a little tire theory for you...

The first three digit number in the tire size is the width of the tire in millimeters. Note 255 is wider than 225. The two digit number in the middle after the slash is the "profile". It's a % of the width.

-255/30 is a 255mm wide tires with a sidewall 30% of the width (76.5mm)
-225/35 is a 225mm wide tire with a sidewall 35% of the width (67.5mm)
As you can see, the 255/30 has a thicker (taller) sidewall than a 225/35.
One other thing you should note is that when trying to stay within 3% of your stock overall diameter, you do not have alot of room to beef up the sidewalls.

All number are subject to some manufacturer's tolerances and the tire inductries lack of standards... but the general idea remains the same.