My preferred tire size calculator:
http://www.wickedbodies.net/Tire-Size-Calculator.htm
Very quick to use.
It shows percent speedometer/odometer readout difference between any two sizes tires.
Also shows what your actual speed is when speedo says "65" (or any speed you enter into the box). Also gives actual tire circumference, total diameter, sidewall height, and percent difference vs. your comparison size.