There's a massive amount of information branded on the sidewall of every tire ... but it's written in code. The tire size is just one small piece of this code. The tire size "P225/45R17 91V" may not mean much to the average person, but to tire geeks like us it speaks volumes. Read on to learn how to crack the tire code with this handy guide dedicated to tire types, sizes, and construction.
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Most tire sizes begin with one or more letters -- for example, P or LT. The letter tells us what type of vehicle or service the tire was designed for.
P-Metric tires are the most common type of tire. The P stands for passenger vehicle, meaning that these tires are designed for use on passenger vehicles like cars, minivans, light-duty pickup trucks (¼ ton or ½ ton capacity), and SUVs.
Metric tires, also known as Euro-Metric tires because the sizing originated in Europe, don't have a letter designation. Euro-Metric sizes are equivalent to P-Metric sizes in dimensions, but Euro-Metric sizes have subtle differences in their load-carrying ratings and capabilities. Euro-Metric tires are usually found on European cars, but they're also used frequently used on SUVs and vans.
Light Truck Metric tires sizes begin with the letters LT. These tires are designed for use on vehicles used to tow trailers or carry heavy loads. This includes SUVs, full-size vans, and medium-duty and heavy-duty pickup trucks with ¾ ton to 1-ton capacity.
These tires are made for light-duty, medium-duty, and heavy-duty pickup trucks (typically ½ ton, ¾ ton or 1-ton load capacity), sport utility vehicles, and vans.
When a tire's descriptor ends in LT, it falls into one of three light truck categories:
Earlier numeric sizes are designed to carry heavy cargo loads and/or tow trailers.
Wide base tires are also designed to carry heavy cargo loads and/or tow trailers on a wheel rim with a diameter of 16.5 inches.
Flotation tires are wider, oversized tires designed to carry heavy cargo loads and/or tow trailers on loose surfaces such as sand, gravel, or dirt.
If the tire size begins with a T, it means that the tire is a temporary spare. Also known as mini spares or space savers, temporary spares are designed for short-term use until the regular tire is repaired or replaced.
Tires beginning with ST are special trailer tires and should only be used on car, boat, or utility trailers.
Euro-Metric tire sizes ending with a C are commercial tires, for use on delivery trucks and vans capable of carrying heavy loads. In addition to the C designation, these sizes are also branded with a load range and service description rating (load range B, C, or D).
The three digits following the service type prefix (if present) tell us the cross-sectional width of the tire in millimeters.
In the example above, the tires width, measured from the widest point of the inner sidewall to the widest point of the outer sidewall when properly mounted, is 225 millimeters. The section width can be converted to inches by dividing the width in millimeters by 25.4 like so: (225 millimeters) / (25.4 mm/in) = 8.86 inches.
The two-digit number that usually follows the tire's section width tells us the aspect ratio, or tire profile measurement.
In this example, the 45 indicates that the sidewall distance, from the wheel rim to the outside of the tread, is 45% of the section width. A lower aspect ratio means a lower-profile tire with a shorter sidewall, while a tire with a higher aspect ratio will have a taller sidewall and look more like a donut. Because we know that the tire size shown in this example has a section width of 8.86 inches and the aspect ratio is 45%, the sidewall height for this tire is 3.98 inches: (8.86 inches) x (.45) = 3.98 inches.
Again using our example tire size from above, the 17 means that the tire should be matched to a 17-inch diameter wheel.
Tires usually come in the following widths (in inches): 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28. Tires in these sizes are typically found on most passenger cars, light-duty light trucks, SUVs, minivans, and vans. Tires with a rim diameter measured in inches are called "inch rim" sizes.
In addition to the inch rim sizes, there are also some unique tire sizes out there. Although not as common, tires are made in half-inch diameters for some heavy-duty light trucks, box vans, and heavy-duty trailers. These sizes are usually 14.5, 15.5, 16.5, 17.5, and 19.5 inches, and an example would be 33x12.5R16.5 118R.
Tires and wheels with unique rim diameters should never be combined with traditional inch rim tires and wheels. Before mounting tires on wheels, the tire and wheel diameters should always be confirmed to match.
When a letter (R, D, or B) follows the two-digit aspect ratio, it tells us the tire's construction. In this example, the R means that the tire has radial construction. Over 98% of all tires sold today are radial tires, where the internal body plies of the tire radiate outward from the center. If there's a D instead of an R, the tire has a bias ply construction, meaning that the internal body plies of the tire crisscross on a diagonal pattern. In belted tires (marked as B), the internal plies crisscross like in a D construction, but there's also an extra layer of reinforcing belts under the tread area. Belted tires are rarely seen these days.
Today, the only speed rating still included in the tire size is the Z rating (sports cars). Since 1991, all other speed ratings are included in the service description, as shown below.
Since 1991, the service description rating is mandatory for all speed ratings (except Z-rated tires) and appears at the end of the tire's size brand. The service description is used to identify the tire's load index (91 in the example above) and speed rating (V in the example).
For more on load indexes and speed ratings, read Breaking the tire code: Understanding tire service description, load index, and speed rating