That info can be important when you are shopping (particularly online) for replacements or you’re looking to upgrade/modify your wheels and tires. While we can’t send you a decoder ring, we can do the next best thing and explain tire size and load rating coding line by line.
For purposes of our discussion, we are going to use P205/55R16 89H as our tire code and we will dissect it character by character to learn what it has to tell us.
A “P” at the start of a tire size indicates the tire was manufactured to certain standards within the U.S. for passenger cars. P is the most common designation and may appear on tires for SUVs and light trucks. LT is the designation for light trucks, vans, and SUVs that have a towing capability. If there is no letter at the beginning of the tire size, it is a Euro metric tire meaning they originated in Europe. You’ll find them on European vehicles but also on many Asian SUVs. The difference between the two is usually a subtle variance in load rating translation. Lastly, the T designation is for temporary and you’ll find it on your compact “donut” spare.
The first three numbers in a tire size refer to the width of the tire measured from sidewall to sidewall in millimeters. In this case, the tire is 205 millimeters wide. This is an important number particularly if you are thinking of putting wider wheels on the rear to potentially get a bigger footprint for the drive wheels.
The aspect ratio tells you the height of the sidewall. It is expressed as a percentage of the tire’s width. So, in this case, the sidewall height is 215 mm x .60 = 112.75 mm or 4.44”. Older and classic cars typically have higher aspect ratios (70 to 90) to accommodate those flashy white walls while performance cars have lower aspects to gain a stiffer sidewall.
The letter after the aspect code tells you the type of construction of the tire. In this instance, the R stands for radial. D stands for diagonal and B stands for Bias. No code means the tire is of cross-ply construction.
The two numbers that follow the construction type describe the diameter of the wheel that the tire is designed to fit, not the diameter of the tire.
The two- or three-digit number after the diameter size is a code that indicates the maximum load (mass or weight) that each tire can carry. There are eighty different ratings ranging from 550 lbs. to 4,080 lbs. so we don’t have room to post them all here. You can find it on the internet. Generally speaking, the higher the code the more weight it can carry. In our sample, the 89 code indicates the tire has a maximum load rating of 1,279 lbs.
The letter at the end of the tire code is the speed rating. This indicates the maximum speed the tire can survive over a period of ten minutes before friction temperatures begin to degrade the tire. There are 32 codes ranging from a ridiculous max speed of 3 mph for an A1 rating to over 186 mph for a (Y) rating. Our sample H rating means the tire is safe to 130 mph.
Not part of the tire size but an important code none the less. The DOT code identifies the company, factory, mold, batch, and date of production (two digits for the week of the year plus two digits for year). This is important for two reasons. First, if there is a recall of a tire, this code is going to tell you if the recall affects your tires. Secondly, when you buy a “new” tire and the DOT code is telling you it has been sitting on the shelf or in a warehouse for 6 years, you may want a newer new tire. Dry rot happens.
So, there you have it. Whether you have performance tires or off-road tires, that’s the rundown on tire size and load ratings. You probably now know more about tire sizing than the clerk at your local discount auto parts store who would be lost without his computer.
