How to read a tire size
P= For Passenger Cars, SUVs, MiniVans & Light Trucks. LT = Heavy Load Trucks, Vans or SUVs. There may not always be a letter in your tire size. A tire starting with "P185" can be replaced with a tire starting with "185" and vice versa. Most websites will not show the starting letter.
The 185 indicates this tire is 185 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel.
Refers to the height of the sidewall (from rim to tread). In our example, the aspect of this tire is 65% of the of 185 millimeter width. The higher the number, the taller/thicker the sidewall.
The "R" in our example indicates this is a radial tire construction. 98% of tires have an "R" construction.
In our example, the diameter (the hole where your rim sits) is 15 inches across.
The load index is how much weight the tire can support. Most vehicles come with a recommended tire load index. SAFETY ALERT: It is NOT recommended that you use a lower load index than the tires that came with your vehicle. You CAN use a higher load index.
The letter refers to the maximum speed the tire can travel and how the tire performs and handles. SAFETY ALERT: You should not "downgrade" the speed rating and all four tires should have the same speed rating.