Tyre Sizing

Passenger Tyre sizing is typically displayed as:

P 215 / 65 R 15 89H

PThe "P" stands for "P-Metric" or "Passenger". Tyres with higher ply ratings will generally start with "LT" which stands for "Light Truck". This indicates the tyre is an LT metric and will always have a Load Range indicated. It is important to note this for vehicles that call for LT metric tyres. Never substitute a P metric tyre for an LT metric tyre, even if all the other dimensions are the same.
215The "215" is the width of a tyre, also known as the "section width". This is the width of the tyre in millimeters at its widest point from sidewall to sidewall when mounted on the recommended rim width. The actual tyre width can vary depending on the rim width it is mounted on.
65The "65" is known as the Aspect Ratio. It is calculated by dividing the section height by the section width and multiplying by 100. (In this example, the sidewall will be 65% of 215).
RThe "R" stands for Radial, meaning it has a radial construction. Radial tyres have ply cords that extend to the beads and are laid at 90 degrees to the centerline of the tread, the carcass being stabilized by a circumferential belt. Other possibilities include "B" for belted construction and "D" for diagonal construction. This means the ply cords extend to the beads and are laid at alternate angles less than 90 degrees to the centerline of the tread.
15The "15" stands for the diameter of the wheel in inches. This is the exact size that this tyre will fit. There are some older rims called "TRX" which are metric measurements like 390. You CAN NOT mix TRX rims with regular tyres or vise-versa.
89The "89" is the load index.
HThe "H" is the speed symbol.