tire buying guide
Trying to read a tire’s size without knowing what you’re looking at is like trying to have a conversation in a foreign language you don’t know. You have to learn the basics.
Fortunately, unlike learning a new language, learning how to read tire size requires far less memorization.
When setting out to buy a new set of tires, it’s important to know what size your tires are before you get started.
This can be tricky since many vehicles can come from the factory with more than one tire size (even in the same model year). However, knowing how to read tire sizes will make the process infinitely easier and arm you with the knowledge you need to make an informed decision. So, let’s get started.
What do the letters and numbers on my tires mean?
Right off the bat, you should know that tire sizing is done using the metric system. Simply put, there are more places in the world that use the metric system, so it has become the standard measurement by which tires are sized around the world.
Next, let’s use P 265 70R17 as our example tire size and 91S as our load index and speed rating. If none of that makes no sense to you, that’s okay. We’ll break it down, piece by piece:
P refers to the vehicle type. In this case, passenger cars. Making matters slightly confusing is the fact that there won’t always be a prefix before the numbers. Regardless, passenger car tires will either have a P or nothing at all. Other vehicle types (light trucks, commercial vehicles, etc.) will always have a prefix. More on that below.
265 refers to the tire’s width. In this example, the width of the tire is 265 mm.
70 refers to the tire’s aspect ratio, which is used to determine the height of the tire. Referred to as a percentage of the width, in this case, the aspect ratio is 70% of 265 mm, or 185.5 mm.
The letter R denotes this is a radial tire – by far the most common type of tire available.
The last number – 17 – refers to the rim diameter. This example tire is meant for a 17-inch rim diameter.
Load Index. Moving onwards from the tire size directly, the number 91 in our example is this tire’s load index (also known as Load Rating). More details on that are below.
Speed Rating. The letter S is our example tire’s speed rating. Again, more on that below.
Vehicle type letter on tire
Passenger car tires will have no prefix or P before the tire size, standing for “passenger” and ¾- and 1-ton light truck specific tires will use the LT prefix (simple enough). But there are other kinds of tire types, too:
A “T” prefix denotes a space-saver or temporary spare.
The “ST” prefix means a Special Trailer tire. Trailer tires are purpose-built and should never be used on vehicles.
The “LT” prefix indicates a tire is designed for light truck duty. However, an “LT” at the end of a tire’s size code (such as 8.75R16.5LT 104/100Q) means that it’s a wide base or flotation tire that’s wide enough to disperse vehicle weight and allow the truck to drive over soft soil or sand without getting stuck.
A “C” at the end of a Euro-metric tire size means that the tire is designed for commercial vehicles such as service trucks or delivery vans.
Metric or Euro-metric tires won’t have a prefix at all. And while most Euro-metric tires are found on European cars, they’re not uncommon on crossovers, pickups, and minivans.
Width number on tires
Width dimensions are also sometimes referred to as “section width” and is typically stated in millimeters. A 225/50R16 tire will have a width of 225 mm; if you want to convert that to inches, there are 25.4 mm in an inch, so divide by 25.4:
225 / 25.4 = 8.86 inches
Aspect ratio is a term that also comes up in engineering, design, and other fields. It refers to the proportion between two uneven sizes, such as the difference between the short sides of a rectangle and the longer sides (expressed as a percentage). In the case of tires, the aspect ratio is the height of the sidewall versus the width of the tread area. This is also referred to as the tire’s “profile.”
Let’s take a 225/50R16 we just talked about; we know that 225 mm is 8.86 inches, and 50% of that 8.86 inches would mean a sidewall, or profile height, of 4.43 inches.
Ever since the 70s, radial tires have been the industry standard, designed with belts that “radiate” out from an imaginary center of the wheel. They’re by far the most common design, making up over 98% of tires sold. Radials are (unsurprisingly) designated with an R, while run-flat tires will have an RF in the code.
While radials are prevalent, bias-ply tires can still be found for specialty applications, industrial tires, ag tires, or trailer tires; they will carry an X, B, or sometimes just a dash in the code.
This is a pretty easy one; diameters will always be expressed in inches on your tire’s sidewall.
Tire size calculator
Let’s say your car no longer has the stock tires, or it’s wearing a different tire size than stock. Or let’s say you want to change the tire size from what you have now. What are you able to do? This is where a tire size calculator comes in.
There are various tire size calculators online that can demystify some of this information, and also includes other tools like revolutions-per-mile and speedometer correction (more on that in a minute). They’re especially useful if you’re considering a slight tweak in tire sizes. Just put in your vehicle’s year/make/model and any other pertinent details that are asked for, and you’ll get the information you need.
Here’s what you need to remember about going up in tire sizes, though. A taller tire or rim will turn slower as it covers the same amount of ground as a smaller tire...or, the other way to think of it is that the bigger tire will cover more ground with each revolution. Either way you cut it, the taller tire is effectively changing the final gear ratio of the vehicle and will throw off speedometer readings. There have been plenty of speeding tickets handed out to drivers who changed tire sizes and thought they weren’t speeding!
In the old days, your speedometer was driven by a cable near the transmission’s output shaft, and recalibrating a speedometer for larger tires involved making some mechanical adjustments (tricky work). Today, vehicles use an electronic speedometer and a different process is involved in recalibrating the speedometer (and you won’t even get your hands dirty in the process). Either way, it’s better than getting stuck with a speeding ticket.
Any GPS-enabled device like a smartphone or nav system will give a speed reading that’s just as accurate as any stock speedometer.
Getting a grasp on tire sizing isn’t tricky, and once you understand what you’re looking at, it starts to make sense. Knowledge is power, as they say, and arming yourself to know what the letters and numbers on your tire means will empower you when it comes time to change or replace them later on.
Nonetheless, the world of tires can be a tricky one to navigate, and if you’re still apprehensive about tackling your next tire purchase solo feel free to give Simple Tire a call. One of our experienced representatives will be happy to guide you through the tire sizing jungle en route to your next tire purchase.
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