Here is a great calculator that will tell you the difference between your current tire and a new tire (size). It automatically calculates the difference in Sidewall Height, Section Width, Overall Diameter, Circumference and Revs per mile. It also tells you the difference in speed, for example, if your current tire goes 65 MPH, with the new tire your actual speed is 68.56 MPH. (See sample screen shot below.)

http://www.discounttire.com/dtcs/infoTireMath.dos