Tires – Size, Load and Speed Ratings
What do you know about your tires? – Tire information series
Tire Size
For safe performance and good tire life it is very important that the tires you put on your vehicle are the right size. On a label found on the door jamb of your car you can check to see what size tires your vehicle was designed to use and on this label you will find some numbers that look like this 225/45 R17.
You will also find these numbers on the sidewall of your tires.
The size of the tire is really pretty simple to decipher. The first number – in this case the 225, is the width of the tread of the tire given in millimeters. The second number describes the height of the sidewall (the part of the tire from the rim of the wheel to the tread). This is expressed as a percentage or ratio of the tread width referred to as the aspect ratio. In this case it is 45 which means that the sidewall height is 45% of 225 millimeters or about 101 millimeters. The last number is the diameter of the wheel or rim expressed in inches – in this case a 17 inch wheel. (I know, I know, it doesn’t make any sense to mix English and metric measurements). The “R” stands for radial construction. A bias ply tire will not have any letter in front of the rim size. Almost all passenger and light truck tires sold today are radial tires. Sometimes you will find a “P” or “LT” in front of the size. This designates what kind of use the tires are intended for. “P” stands for passenger tire and “LT” stands for light truck tire. Sometimes you will see a ZR instead of an R. The “Z” designates the speed rating of the tire. Speed ratings will be explained next.
Load and Speed Ratings
It isn’t just the size of the tire that is important. Load ratings and speed ratings are also very important for safe driving and good tread life. Again the manufactures recommendation will be found on the door jamb sticker and somewhere on the side of the tire you will see numbers and letters in this format “94H”. The numbers represent the load rating of the tire. Generally the higher the number the more weight the tire is designed to carry. In this case the “94” indicates the tire is designed to carry up to 1477 lbs. at a normal inflation pressure. “H” means that this tire is rated for speeds up to 130 mph. Light truck tires have a different weight rating classification system with letters such as B,C,D,E which correspond to a ply rating. “C” stands for 6 ply tires, “D” for 8 ply, and “E” for 10 ply. These different ratings indicate the relative strength of the tire and maximum inflation pressures the tire can safely handle.
If you are located in the Harrisonburg Virginia area – give us a call and we will help you choose the right tires for your car or light truck.
