View Here : Tire Size Meaning
There are, however, online tire size calculators that help you compute whether the tires have similar overall diameters. The rule is, if you want to change your tire size, they should not have a difference in diameter of more than 1%, ideally, it should be less than that.
Tire Class – “P”: The first character in a tire size is usually a letter, which designates the tire’s class. In this particular example, a “P” indicates that the tire is a “passenger” car tire.