Tire Size Calculations
 

Here it is:
P245/45R17
245 = Width (in millimeters)
45 = Aspect (percent of width [here: 45% of 245 mm])
17 = Rim Size (in inches)

(((Width/25.4)x(Aspect/100))x2)+Rim

Example:
P245/45R17
(((245/25.4) x (45/100)) x 2) + 17

Run that through the calculator and you can use the analyzer with more accuracy. I originally thought that my tires were 26.5 inches when they were only 25.6 - see what research does...