What do the numbers on tires mean?
There’s a massive amount of information branded on the sidewall of every tyre … but it’s written in code. The tyre size is just one small piece of this code. The tyre size “P225/45R17 91V” may not mean much to the average person, but to tyre geeks like us it speaks volumes. Read on to learn how to crack the tyre code with this handy guide dedicated to tyre types, sizes, and construction.
Tyre service type ratings
Most tyre sizes begin with one or more letters — for example, P or LT. The letter tells us what type of vehicle or service the tyre was designed for.
P = P-Metric (Example: P215/65R17 98T)
P-Metric tyres are the most common type of tyre. The P stands for passenger vehicle, meaning that these tyres are designed for use on passenger vehicles like cars, minivans, light-duty pickup trucks (¼ ton or ½ ton capacity), and SUVs.
Metric / Euro-Metric (Example: 185/65R15 88T)
Metric tyres, also known as Euro-Metric tyres 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 tyres are usually found on European cars, but they’re also used frequently used on SUVs and vans.
LT – Light Truck (prefix) (Example: LT235/75R15 104/101S/C)
Light Truck Metric tyres sizes begin with the letters LT. These tyres 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.
LT – Light Truck (suffix) (Example: 9.5-16.5 LT121/117R)
These tyres 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 tyre’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 tyres are also designed to carry heavy cargo loads and/or tow trailers on a wheel rim with a diameter of 16.5 inches.
Flotation tyres are wider, oversized tyres designed to carry heavy cargo loads and/or tow trailers on loose surfaces such as sand, gravel, or dirt.
T = Temporary Spare (Example: T145/70R17 106M)
If the tyre size begins with a T, it means that the tyre is a temporary spare. Also known as mini spares or space savers, temporary spares are designed for short-term use until the regular tyre is repaired or replaced.
ST – Special Trailer (Example: ST175/80R13)
Tyres beginning with ST are special trailer tyres and should only be used on car, boat, or utility trailers.
C = Commercial (Example: 31×10.50R15/ C109R)
Euro-Metric tyre sizes ending with a C are commercial tyres, 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).
Tyre Sizes Explained
Section width of tyre (Example: P 225/45R17 91V)
The three digits following the service type prefix (if present) tell us the cross-sectional width of the tyre in millimeters.
In the example above, the tyres 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.
Aspect ratio of sidewall (Example: P225/ 45R17 91V)
The two-digit number that usually follows the tyre’s section width tells us the aspect ratio, or tyre 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 tyre with a shorter sidewall, while a tyre with a higher aspect ratio will have a taller sidewall and look more like a donut. Because we know that the tyre size shown in this example has a section width of 8.86 inches and the aspect ratio is 45%, the sidewall height for this tyre is 3.98 inches: (8.86 inches) x (.45) = 3.98 inches.
Tyre and wheel diameter (Example: P225/45R 1791V)
Again using our example tyre size from above, the 17 means that the tyre should be matched to a 17-inch diameter wheel.
Tyres usually come in the following widths (in inches): 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28. Tyres in these sizes are typically found on most passenger cars, light-duty light trucks, SUVs, minivans, and vans. Tyres with a rim diameter measured in inches are called “inch rim” sizes.
Unique wheel diameters
In addition to the inch rim sizes, there are also some unique tyre sizes out there. Although not as common, tyres 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 33×12.5R16.5 118R.Tyrebf
Tyres and wheels with unique rim diameters should never be combined with traditional inch rim tyres and wheels. Before mounting tyres on wheels, the tyre and wheel diameters should always be confirmed to match.
More info included in the tyre size
Internal construction (example: P225/45 R17 91V)
When a letter (R, D, or B) follows the two-digit aspect ratio, it tells us the tyre’s construction. In this example, the R means that the tyre has radial construction. Over 98% of all tyres sold today are radial tyres, where the internal body plies of the tyre radiate outward from the center. If there’s a D instead of an R, the tyre has a bias ply construction, meaning that the internal body plies of the tyre crisscross on a diagonal pattern. In belted tyres (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 tyres are rarely seen these days.
Speed rating (Example: P225/45R17 91 V)
Today, the only speed rating still included in the tyre size is the Z rating (sports cars). Since 1991, all other speed ratings are included in the service description, as shown below.
Service description rating (Example: P225/45R17 91V)
Since 1991, the service description rating is mandatory for all speed ratings (except Z-rated tyres) and appears at the end of the tyre’s size brand. The service description is used to identify the tyre’s load index (91 in the example above) and speed rating (V in the example).