Deciphering Your Tire. The Tire Size
To the lay person, the markings of a tire can look like gibberish. That’s not to say that every person in the business actually has a firm grasp on everything stamped onto a tire either.
The purpose of this post will be to describe and dissect a tire size and show how it fits together to give us its dimensions.
There are several ways to size a tire including numeric, alpha numeric, floatation, and metric. Since the first two are almost exclusively used on antique vehicles, we will discuss only floatation and metric sizing.
I must admit, I love floatation sizing. Why, because it is so simple. The tire is 31 inches tall, 10.50 inches wide, it fits on a 15 inch wheel, and it’s for light trucks.
Metric sizing on the other hand is a bit more cryptic. However, with a little knowledge of how the sizing works and what the numbers and letters mean in the size, you can decipher quite a lot about the makeup of the tire in question.
P-stands for Passenger commonly referred to as P-Metric. This is used in the US to determine its application. There is also LT for Light Truck, T for Temporary, and ST for Service Trailer. Some tires will have no designation at all. These tires are Euro-metric size tires and they will look like the same size, for example 225/60R16 (no P). However, they aren’t. They will have slightly different load capacities.
225–This is a measurement of the widest part of the tire in millimeters excluding scuff guards, rim protectors, etc. This is not necessarily the tread width of the tire, but its widest section.
Aspect Ratio or Profile
60–This is the Aspect Raito of the tire. In other words, the ratio of sidewall compared to the section width.
16–this is the diameter of the wheel. In this case, 16 inches.
R-Radial This tells of us its construction. Nowadays, almost every tire is of radial [R] construction.
98T-Service Description This includes the Load Index (98) and the Speed Rating (T). This tells us how much it can carry and how fast you can go on the tire.
Since the main point of this post is to talk about the size of the tire, I’ll leave the construction, load index and speed rating to a later post.
Ok, so how do these numbers go together to form the tire’s dimensions? This may not seem like a very relevant topic to the average consumer who just wants new tires and will never deviate from the factory size. However, it is relevant if you plan on installing a new set of larger diameter wheels or just want to put on a bigger tire.
This formula will tell you how tall a tire is. Overall Diameter–(Section Width X Aspect Ratio X 2)/25.4+Wheel Diameter. By converting the section width to inches (dividing by 25.4) we know its width in inches(225/25.4=8.86). We can even see what the length of an individual sidewall is with a part of this formula ((225×60%)/25.4)=5.31 inches. In this case the tire is 26.62 inches tall. 5.31+5.31+16=26.62.
I’ve heard over the years from many people so many incorrect assumptions of what the components of a metric tire size are. Hopefully this explains how a tire’s size isn’t just arbitrary, but actually usable data that converts to measurable dimensions of a tire.