Close up of tire size markings and tread

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How to read tire size?

When setting out to buy a new set of tires, it’s important to know what size your tires actually are before you even get started. And, that it can be tricky since many vehicles can come from the factory with more than one tire size (even in the same model year).

At SimpleTire, we’ve already covered what the numbers on the sidewall mean but hope to add more clarity on how to read tire size specifically.

Tire type

Passenger car tires will have the P prefix before the tire size, standing for “passenger” and light truck 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.

Tire width

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

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 that 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.

Internal construction

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.

Diameter

This is a pretty easy one...diameters will always be expressed in inches on your tire’s sidewall.

There’s other handy information on the sidewall, including speed rating, load rating, serial number, and DOT codes, that’s important to know too.

Tire size calculators

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 that is that it 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 speedo 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 speedo (and you won’t even get your hands dirty in the process). Either way, it’s better than getting stuck with a speeding ticket.

SimpleTip Any GPS-enabled device like a smartphone or nav system will give a speed reading that’s just as accurate as any stock speedometer.

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