just measure where you want it. Tire sizes are easy to read...
example: a 275/60R15 =
275mm wide (tread area)
60% of 275mm is the profile/series (height of the sidewall)
275x60 = 165mm
R = Radial LT= Light Truck etc
15 inch wheel
So just measure up how wide and tall you want the tire with whatever offset you have and you'll have the perfect tire fit