Common Tire Vocabulary Explained: From Sidewall Numbers to Rims and Wide Tires

The numbers on a tire sidewall describe the tire's size and its performance limits. In a typical code such as 225/45R17 91V, 225 is the section width in millimetres, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, 17 is the rim diameter in inches, 91 is the load index, and V is the speed rating. This glossary explains each of these terms in plain language. It shows how to calculate what the size numbers mean for sidewall height, overall diameter, and circumference, and it explains related words such as rim, wheel, tread depth, and wide tires.

How to Read the Numbers on a Tire Sidewall

A standard passenger tire size combines three kinds of information. It gives dimensions (width and rim diameter), a ratio (aspect ratio), and performance ratings (load index and speed rating). The units are mixed: width is in millimetres, rim diameter is in inches, and the aspect ratio is a percentage with no unit at all.

Marking (example 225/45R17 91V)	Name	Type of value	Unit	What it tells you
225	Section width	Dimension	Millimetres	The nominal width of the tire from sidewall to sidewall
45	Aspect ratio	Ratio	Percent of section width	The sidewall height is 45% of 225 mm
R	Construction type	Code letter	None	Radial construction, the standard for modern road tires
17	Rim diameter	Dimension	Inches	The tire fits a wheel with a 17-inch rim diameter
91	Load index	Performance rating (code)	Code for kilograms	Maximum load per tire: 615 kg
V	Speed rating	Performance rating (code)	Code for km/h	Maximum rated speed: 240 km/h (about 149 mph)

Section width

Section width is the nominal width of a tire, measured in millimetres from the outside of one sidewall to the outside of the other. It does not include raised lettering or protective rim ribs. In 225/45R17, the section width is 225 mm. Section width is not the same as tread width, which is the width of the part that touches the road. Tread width is usually narrower than section width.

Aspect ratio

The aspect ratio is the height of the sidewall expressed as a percentage of the section width. In 225/45R17, an aspect ratio of 45 means the sidewall is 45% as tall as the tire is wide: 0.45 × 225 mm = 101.25 mm. A lower number means a shorter sidewall, which is why tires with aspect ratios around 30 to 45 are often called low-profile tires. Because the aspect ratio is a percentage, two tires with the same aspect ratio but different widths have different sidewall heights.

Construction letter

The letter between the aspect ratio and the rim diameter describes how the tire's internal plies are laid out. R means radial, the construction used by almost all modern car tires. D or a dash indicates bias-ply (diagonal) construction, which is still found on some trailer, vintage, and specialty tires. In some performance tire sizes the letters ZR appear, for example 245/40ZR18. ZR marks a radial tire rated for speeds above 240 km/h.

Rim diameter

Rim diameter is the diameter of the wheel the tire is designed to fit, measured in inches at the bead seat where the tire sits on the rim. It is not measured at the outer lip of the wheel. In 225/45R17, the rim diameter is 17 inches, which is 431.8 mm. A tire must match the wheel's rim diameter exactly. A 17-inch tire cannot be mounted on a 16-inch or 18-inch wheel.

Load index

The load index is a number code for the maximum weight a single tire can carry when it is inflated to the pressure specified for that load. The number is not the weight itself; you look it up in a standard table. Higher numbers mean higher capacity.

Load index	Maximum load per tire
85	515 kg
88	560 kg
91	615 kg
94	670 kg
95	690 kg
100	800 kg

Some light-truck tires show two load indexes, such as 123/120. The first number is the rating when the tire is used singly (123 = 1,550 kg). The second is the rating per tire when it is used in a dual-wheel pair (120 = 1,400 kg). XL (extra load) or Reinforced on a passenger tire means it is built to carry a higher load than the standard version of the same size, at a higher inflation pressure.

Speed rating

The speed rating is a letter code for the maximum speed a tire is certified to sustain when it is properly inflated and carrying a load within its load index. It describes the tire's tested capability. It is not a recommended driving speed.

Speed rating	Maximum speed
Q	160 km/h (99 mph)
R	170 km/h (106 mph)
S	180 km/h (112 mph)
T	190 km/h (118 mph)
H	210 km/h (130 mph)
V	240 km/h (149 mph)
W	270 km/h (168 mph)
Y	300 km/h (186 mph)

The load index and speed rating together are called the service description, written for example as 91V.

How to Calculate Sidewall Height, Overall Diameter, and Circumference

You can calculate a tire's nominal sidewall height, overall diameter, and circumference from its size code using simple arithmetic. Because the code mixes millimetres and inches, first convert the rim diameter to millimetres (1 inch = 25.4 mm).

1. Sidewall height = section width × (aspect ratio ÷ 100). For 225/45R17: 225 × 0.45 = 101.25 mm.
2. Rim diameter in millimetres = rim diameter in inches × 25.4. For a 17-inch rim: 17 × 25.4 = 431.8 mm.
3. Overall diameter = rim diameter + (2 × sidewall height), because there is sidewall above and below the wheel. For 225/45R17: 431.8 + 202.5 = 634.3 mm (about 24.97 inches).
4. Circumference = overall diameter × π (about 3.1416). For 225/45R17: 634.3 × 3.1416 ≈ 1,992.7 mm.
5. Revolutions per kilometre (optional) = 1,000,000 mm ÷ circumference. For 225/45R17: 1,000,000 ÷ 1,992.7 ≈ 502 revolutions.

How Do Width, Aspect Ratio, and Rim Diameter Affect Tire Size Comparisons?

Each of the three size numbers changes a tire's geometry differently. A wider tire with the same aspect ratio gets a taller sidewall, because the aspect ratio is a percentage of the width. A lower aspect ratio shortens the sidewall. A larger rim diameter adds directly to the overall diameter. When people move to a larger wheel (often called plus-sizing), they usually lower the aspect ratio so that the overall diameter stays close to the original.

Tire size	Sidewall height	Overall diameter	Circumference	Diameter difference vs 225/45R17
205/55R16	112.8 mm	631.9 mm	1,985.2 mm	−2.4 mm (−0.4%)
225/45R17 (reference)	101.3 mm	634.3 mm	1,992.7 mm	—
225/40R18	90.0 mm	637.2 mm	2,001.8 mm	+2.9 mm (+0.5%)
245/40R18	98.0 mm	653.2 mm	2,052.1 mm	+18.9 mm (+3.0%)

The table shows three practical patterns:

• Different numbers, similar diameter. 205/55R16, 225/45R17, and 225/40R18 have very different-looking codes, but their calculated overall diameters fall within about 5 mm of each other. The larger wheel is offset by a shorter sidewall.
• Width changes sidewall height. 225/40R18 and 245/40R18 share the same aspect ratio and rim size. The 245 tire's sidewall is still 8 mm taller (98 mm vs 90 mm), because 40% of 245 is more than 40% of 225.
• Small percentages add up. A 3.0% larger diameter means about 3.0% more distance per wheel revolution. If a speedometer is calibrated for the 225/45R17 tire, a 245/40R18 tire would, by calculation alone, make the true speed about 103 km/h when the speedometer shows 100 km/h.

Calculated geometry vs actual tire dimensions

Calculated figures are nominal values derived from the size code, not exact measurements of a real tire. Actual dimensions differ for several reasons:

• Measuring rim width. Section width is defined on a specific measuring rim width. On a narrower or wider wheel, the same tire will measure narrower or wider.
• Manufacturer and model differences. Two tires with the same size code from different brands, or even different models from the same brand, can differ by a few millimetres in width and diameter. Manufacturers publish the actual specifications for each model.
• Tread depth. A new tire is taller than a worn one, so overall diameter shrinks slightly as the tread wears.
• Load and inflation. Under the weight of a vehicle, the tire flattens where it meets the road. The loaded radius is therefore smaller than half the calculated diameter, and it varies with inflation pressure.

Geometry vs vehicle fitment compatibility

A size calculation shows how two tires compare on paper. It does not show whether a tire is suitable for a particular vehicle. Fitment also depends on clearance to suspension parts, fenders, and brake components at full steering lock and suspension compression. It depends on the rim width range approved for the tire, wheel offset, a load index and speed rating at least equal to the vehicle's requirement, and how the vehicle's speedometer and stability systems respond to a change in rolling circumference. The approved tire sizes for a vehicle are listed on its tire placard (usually on the driver's door jamb or fuel flap) and in the owner's manual.

Other Common Sidewall Markings

Beyond the size and service description, most tire sidewalls carry several other markings:

• Maximum inflation pressure, for example MAX PRESS 350 kPa (51 psi). This is the highest pressure the tire is designed to hold. It is not the recommended driving pressure. The correct inflation pressure for everyday use is set by the vehicle manufacturer and printed on the vehicle's tire placard.
• Maximum load, stated in kilograms and pounds. It matches the load index.
• DOT code (in the United States and on many exported tires). This code ends in four digits that give the week and year of manufacture. For example, 2324 means the 23rd week of 2024.
• UTQG ratings (US market). These ratings cover treadwear (a relative number), traction (AA, A, B, C), and temperature resistance (A, B, C).
• M+S (mud and snow). The tread design meets a geometric definition for mud and snow use.
• 3PMSF (three-peak mountain snowflake symbol). The tire has passed a standardised snow traction test.
• Tubeless and Run-flat markings. These describe whether the tire needs an inner tube, and whether it can be driven for a limited distance after losing pressure.

Other Tire Size Formats

Not every tire size uses the format 225/45R17. Common variations include:

• P-metric, for example P225/45R17. The P stands for passenger car, a designation used mainly in North America. Metric sizes without a prefix (often called Euro-metric) use slightly different load tables.
• LT-metric, for example LT265/75R16 123/120R. LT stands for light truck. These tires are built for heavier loads and often carry a load range letter such as E.
• Flotation sizes, for example 33x12.50R15. These are used on off-road and light-truck tires and are given entirely in inches: 33 inches overall diameter, 12.50 inches section width, and a 15-inch rim diameter. In this format the overall diameter is stated directly instead of being calculated.

What Are Wide Tires Called?

Wide tires are most often simply called wide tires, but several more specific names exist depending on the vehicle and context:

• Low-profile tires are tires with a low aspect ratio. They are often wide, though the term describes sidewall height rather than width.
• Fat tires is an informal term for any noticeably wide tire. It is also the standard name for the very wide tires on fat bikes.
• Flotation tires are wide, large-volume tires that spread a vehicle's weight over a bigger contact area. They are used on off-road vehicles and agricultural machinery.
• Wide-base tires (also called wide-base singles or super singles) are used on heavy trucks and trailers, where one wide tire replaces a pair of dual tires. An example size is 445/50R22.5.
• Staggered fitment is not a tire type but a setup. In a staggered fitment, the rear tires are wider than the front tires, which is common on rear-wheel-drive performance cars.

On a sidewall, a tire's width is shown only by the section width number. A 275 or 295 is wide for a passenger car, but there is no official cutoff that makes a tire wide.

What Is the Difference Between Rims and Tires?

A tire is the rubber component that contacts the road. A rim is the outer edge of the metal wheel that holds the tire. The tire is a flexible casing made of rubber compounds, fabric plies, and steel belts. It contains the air and provides grip and cushioning. The rim is the part of the wheel where the tire's beads seat and seal. The wheel is the whole metal assembly: the rim plus the centre section (disc or spokes) that bolts to the vehicle's hub.

Term	What it is	Main material	How its size is stated
Tire	Inflatable casing that contacts the road	Rubber, fabric, steel	Size code, e.g. 225/45R17
Rim	Outer part of the wheel where the tire seats	Steel or aluminium alloy	Diameter and width in inches, e.g. 17 x 7.5J
Wheel	Complete rim plus centre section mounted to the hub	Steel or aluminium alloy	Rim size plus bolt pattern, centre bore, and offset

In everyday speech, people often say rims when they mean the whole wheel, especially for custom alloy wheels. The link between tire and rim on the sidewall is the rim diameter number. A tire marked R17 fits only a 17-inch rim, and each tire size also has a range of approved rim widths.

Words Related to Tire: A Glossary of Tire Parts and Terms

These are the most common words related to tire construction, wear, and fitment, each defined in plain language.

Tread

The patterned outer surface of the tire that contacts the road and provides grip.

Tread depth

The depth of the grooves in the tread, measured in millimetres or in 32nds of an inch. Tread depth decreases as the tire wears. Many countries set a legal minimum, such as 1.6 mm in the European Union and 2/32 inch in many US states.

Tread wear indicators

Small raised bars at the bottom of the main grooves. When the tread wears down level with these bars, the tire has reached its minimum usable depth.

Groove

A main channel in the tread that drains water away from the contact patch.

Sipe

A thin slit cut into a tread block to add biting edges for wet, snow, and ice grip.

Rib

A continuous band of tread running around the tire's circumference.

Shoulder

The area where the tread meets the sidewall.

Sidewall

The side of the tire between the tread and the rim. The sidewall carries the size code and other markings.

Bead

The reinforced inner edge of the tire, usually containing steel wire. The bead seals against the rim.

Casing (carcass)

The tire's structural body, made of fabric plies that give it shape and strength.

Belts

Layers of steel or fabric under the tread that stabilise it and resist punctures.

Contact patch

The area of tread actually touching the road at any moment.

Inflation pressure

The air pressure inside the tire, measured in kPa, bar, or psi. Recommended pressure comes from the vehicle manufacturer; maximum pressure comes from the tire sidewall.

Valve stem

The small valve on the wheel used to add or release air.

Inner tube

A separate inflatable tube used inside tube-type tires. Most modern car tires are tubeless.

Tire size

The full size code describing section width, aspect ratio, construction, and rim diameter.