ok, not to knock any one, but i work 6 days a week in a service shop, ( oil changes, tires, all the other bullshit) tire sizes go like this... take note this is an example for and extremely common size tire...
P235/75/R15
P is for passenger, or the standard load range... safe PSI varies from 35psi on older tires all the way to 50psi on newer tires
235 is the width of the tread in mm, so its 235mm wide, or 23.5 cm
75 is the percentage of the width as the sidewall height... I.e 235x75%= 176.25 mm of side wall height
R15 is the Radius of the rim in inches... so R16 is 16 inch wheel, R17 is 17 inch wheel and so on...
if there is a letter or letters after or sometimes before the size that is the speed rating of the tire...
if i left anything out let me know and i'll edit it up...