Tire-size markings: width, aspect ratio, construction and rim size

A tire size such as P225/45R17 91V is a compact code. P means the tire is a passenger-car tire, 225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of that width), R means radial construction, and 17 is the diameter of the wheel rim it fits, in inches. The 91 and V that follow are the load index and speed rating, together called the service description. Width and sidewall are metric while rim diameter is in inches, so one code mixes two unit systems. Some off-road, light-truck, ATV and older motorcycle tires use different inch-based codes, covered further down.

P  225 / 45  R  17   91  V
|   |    |   |   |    |   |
|   |    |   |   |    |   +-- speed rating
|   |    |   |   |    +------ load index
|   |    |   |   +----------- rim diameter (inches)
|   |    |   +--------------- construction (R = radial)
|   |    +------------------- aspect ratio (%)
|   +------------------------ section width (mm)
+---------------------------- tire type (P = passenger)

What each part of the metric code means

Tire type prefix or suffix

The leading letter identifies the tire's intended service. P marks a P-metric passenger tire, LT a light-truck tire, T a temporary spare and ST a special trailer tire. A size with no prefix, such as 225/45R17, is usually a Euro-metric tire. A C after the rim diameter on a Euro-metric size (for example 195/70R15C) marks a commercial tire built for heavier loads. The prefix matters because tires of the same dimensions can be rated for different loads depending on type.

Section width

The first number is the section width: the distance from the outside of one sidewall to the outside of the other at the widest point, in millimeters. It excludes raised lettering and protective ribs, and it is defined with the tire mounted on a specified rim width. On a wider or narrower rim, the same tire measures somewhat wider or narrower.

Aspect ratio

The number after the slash is the aspect ratio, also called the profile or series. It is the sidewall height expressed as a percentage of the section width, not a measurement in its own right. A 225/45 tire has a sidewall 45 percent as tall as its 225 mm width, or about 101 mm. A lower number means a shorter sidewall relative to width. Because it is a percentage, the same aspect ratio gives a taller sidewall on a wider tire.

Construction letter

The letter before the rim diameter describes how the tire's internal plies are laid. R means radial, where the plies run across the tire at right angles to the direction of travel. This is the standard construction for modern passenger cars. D means diagonal (bias-ply), with plies crossing at an angle, and B means bias-belted. On some inch-based and off-road sizes, a hyphen in place of a letter indicates bias construction.

Rim diameter

The last number in the size proper is the diameter of the wheel the tire is designed to mount on, in inches. A tire marked 17 fits only a 17-inch wheel. Rim diameter must match exactly. Width and aspect ratio can sometimes vary within limits.

How the numbers combine: sidewall height and overall diameter

Width, aspect ratio and rim diameter together set the tire's geometry. Multiplying width by aspect ratio gives sidewall height. Overall diameter is the rim diameter plus two sidewalls, one above the rim and one below. Since sidewalls are calculated in millimeters and rims are sized in inches, one value must be converted (25.4 mm per inch). Online tire diameter calculators perform this same arithmetic.

Sidewall height (mm)   = section width (mm) x aspect ratio / 100
Sidewall height (in)   = sidewall height (mm) / 25.4
Overall diameter (in)  = rim diameter (in) + 2 x sidewall height (in)

Example: 225/45R17
Sidewall height        = 225 x 45 / 100 = 101.25 mm = 3.99 in
Overall diameter       = 17 + 2 x 3.99  = 24.97 in

The table applies the formula to three common sizes. It shows how a larger rim can be paired with a lower aspect ratio to keep overall diameter close to the original. Tire shops call this plus-sizing.

Size | Sidewall height | Nominal overall diameter

205/55R16 | 112.75 mm (4.44 in) | 24.88 in

225/45R17 | 101.25 mm (3.99 in) | 24.97 in

225/40R18 | 90.00 mm (3.54 in) | 25.09 in

Overall diameter matters in use. A larger-diameter tire travels farther per revolution, so the speedometer and odometer, calibrated for the original size, read lower than true speed and distance. A smaller tire has the opposite effect. Sidewall height also affects ride and handling: a shorter sidewall generally flexes less, which tends to sharpen steering response but leaves less cushioning between the road and the wheel.

These figures are nominal design dimensions. The real diameter and width of a mounted tire vary with the manufacturer and model, rim width, inflation pressure, load and tread wear. Treat calculator output as a comparison tool, not an exact measurement. If precise clearance matters, use the tire maker's published specifications for the specific model.

Load index and speed rating

The service description after the size, such as 91V, gives two separate ratings. The number is the load index, a code for the maximum load one tire can carry at its specified inflation. It is not a weight in itself; it must be looked up on a load-index chart. The letter is the speed rating, the maximum sustained speed the tire is certified for when correctly inflated and loaded.

Load index | Maximum load per tire

91 | 615 kg (1,356 lb)

95 | 690 kg (1,521 lb)

100 | 800 kg (1,764 lb)

These are selected entries only. Use a full load-index chart from a tire manufacturer for any other value. P-metric and Euro-metric tires of the same size and load index may not be rated identically. Tires marked XL (extra load) or reinforced are built to carry more at a higher inflation pressure. Light-truck tires usually show a load range letter (such as C, D or E) indicating their ply rating.

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)

ZR in the size, as in 245/40ZR18 97Y, is an older way of marking a tire rated for sustained speeds above 240 km/h (149 mph). The Z sits next to the R for radial construction. Modern ZR tires normally also carry a service description stating the actual speed rating. A rating shown in parentheses, such as (Y), indicates capability beyond the top speed of that letter, with the exact limit set by the tire maker.

Inch-based sizes: flotation, light-truck and ATV tires

Many off-road and light-truck tires use an all-inch flotation size such as 33x12.50R15LT. Here the first number is the approximate overall diameter (33 in), the second is the section width (12.50 in), R is radial construction, 15 is the rim diameter in inches, and LT marks a light-truck tire. Overall diameter is stated directly, so no calculation is needed. To compare with a metric size, convert the metric tire's diameter using the formula above.

ATV and UTV tires commonly follow a similar pattern, such as 25x8-12. That reads as about 25 inches tall, 8 inches wide, on a 12-inch rim. The hyphen typically indicates bias construction, while an R in that position (25x8R12) indicates radial. Some ATV tires carry a prefix such as AT. Manufacturers also publish metric equivalents for some sizes.

Motorcycle tire sizes

Most modern motorcycle tires use the same metric logic as cars. In 120/70ZR17 58W, 120 is the width in millimeters, 70 the aspect ratio, ZR the high-speed radial marking, 17 the rim diameter in inches, and 58W the load index and speed rating. Front and rear tires are usually different sizes and are not interchangeable. Some older, cruiser and off-road motorcycle tires use inch-based codes (width in inches followed by rim diameter, such as 3.00-18) or letter-based alphanumeric codes. The supplied sources do not detail those systems, so check the tire maker's size chart or the motorcycle's documentation to interpret or convert them. Bicycle tires use a separate sizing system that is outside the scope of this guide.

How to find and confirm the correct size for your vehicle

• 1. Find the vehicle maker's specified size. On US-market vehicles it is printed on the Tire and Loading Information placard, usually on the driver's door jamb, and in the owner's manual. Some vehicles specify different front and rear sizes, or list several approved sizes depending on trim and wheel option.
• 2. Read the size molded into the sidewall of the tires currently fitted. Compare it with the placard, since a previous owner or shop may have installed a different size.
• 3. Record the full code, including prefix, construction letter, load index and speed rating, not just the three main numbers.
• 4. When choosing replacements, match the specified size. As general guidance, choose a load index and speed rating at least equal to the original specification. Confirm this against the vehicle maker's documentation, as requirements differ between vehicles and markets.
• 5. If you are considering a different size, compare nominal overall diameters with the formula above. Also confirm rim width compatibility, body and suspension clearance, and load capacity with a qualified tire professional before buying. A size that fits one configuration of a model may not fit another.
• 6. Leave mounting and balancing to a qualified tire shop. Seating a tire bead on a rim requires proper equipment and can be hazardous.
• 7. Completion check: the size and service description on the new tires match the placard (or a size the vehicle maker or tire professional has confirmed). Tires are inflated to the pressure on the vehicle placard. The maximum pressure molded into the sidewall is the tire's limit, not the recommended running pressure.

What the size code does not tell you

The size marking describes dimensions, construction and ratings. It does not indicate the tire's age, tread compound, wear rate or which sizes are approved for a particular vehicle. Other sidewall markings, such as the DOT identification code and treadwear grades, cover some of that information but are separate from the size. The load and speed tables above are partial, and nominal diameters are calculated rather than measured. For exact capacity, dimensions or fitment on a specific model year and trim, rely on the vehicle placard, the owner's manual and the tire manufacturer's published data.