Car tires are described by an alphanumeric code which is generally molded into the side-wall of the tire. This code specifies the dimensions of the tire & some of its key limitations such as load bearing ability & maximum speed. Sometimes the inner sidewall contains information not included on the outer sidewall - & vice versa.

The code has grown in complexity over the years as is evident from the mix of metric & imperial units & ad-hoc extensions to lettering & numbering schemes. New automotive tires frequently have ratings for traction, treadwear, & temperature resistance (collectively known as The Uniform Tire Quality Grade (UTQG) ratings).

The meaning of tire codes

The tire code consists of a string of letters & numbers, as follows: An optional letter (or letters) indicating the intended use or vehicle class for the tire 'P' - Passenger 'LT' - Light Truck 'ST' - Special Trailer 'T' - Temporary The width of the tire - in millimeters, from sidewall edge to sidewall edge. A slash character ('/'). The ratio of the sidewall height to the total width of the tire - as a percentage. If this is missing, it is assumed to be 82%. If the number is larger than 200, then this is the diameter of the entire tire in millimeters. Construction of the fabric of the tire 'B' - bias belt 'D' - diagonal 'R' - radial Diameter of the rim that this tire is designed to fit - in inches. Load index. Speed rating - one or perhaps two letters (see table below). Additional marks. Load Index Load Index Code Pounds Kilograms   Code Pounds Kilograms 71 761 345   91 1,356 615 72 783 355   92 1,389 630 73 805 365   93 1,433 650 74 827 375   94 1,477 670 75 853 387   95 1,521 690 76 882 400   96 1,565 710 77 908 412   97 1,609 730 78 937 425   98 1,653 750 79 963 437   99 1,709 775 80 992 450   100 1,764 800 81 1,019 462   101 1,819 825 82 1,047 475   102 1,874 850 83 1,074 487   103 1,929 875 84 1,102 500   104 1,984 900 85 1,135 515   105 2,039 925 86 1,168 530   106 2,094 950 87 1,201 545   107 2,149 975 88 1,235 560   108 2,205 1,000 89 1,279 580   109 2,271 1,030 90 1,323 600   110 2,337 1,060

Speed rating codes
The code is made up of one or two letters describing the maximum safe speed for the tire.

The code "ZR" means a speed limit for the tire of more than 240 km/h. In this case there is often an additional code, example: 225/45 ZR 17 Y.

Additional marks
There are numerous other markings on a typical tire, these may include:

M&S: On winter tires, at the end of the above mentioned codes the following abbreviation can be found: M&S for Mud & Snow. On spike tires there is an additional E after the S.
E4 - Tire approved according ECE-regulations, (the number indicates the country of approval).
030908 - Approval number of the tire
DOT code: All tires imported into the USA have the DOT code, as required by the Department of Transportation (DOT). It specifies the company & factory, mold, batch, & date of production (2 digits for week of the year plus 2 digits for year; or 2 digits for week of the year plus 1 digit for year for tires made prior to 2000)
TL - Tubeless
TT - Tubetype, tire with an inner-tube
Made in - Country of production
C (commercial) - Tires for light trucks (Example: 185 R14 C)
B - Bias Belted tires for motorcycles (Example: 150/70 B 17 69 H)=diagonal construction with belt added under the tread
SFI - abbr. for "side facing inwards" = inside of asymmetric tires
SFO - abbr. for "side facing outwards" = outside of asymmetric tires
TWI - Tire wear index, an indicator in the main tire profile which shows when the tire is worn down & needs to be replaced
SL (standard load): Tire for normal usage & loads
XL (extra load): Tire for heavy loads
rf - Reinforced tires
Arrows - Some tread designs are designed to perform better when driven in a specific direction (clockwise or counter-clockwise). Such tires will have an arrow showing which way the tire should rotate when the vehicle is moving forwards. It is important not to put a 'clockwise' tire on the left hand side of the car or a 'counter-clockwise' tire on the right side.

Tire geometry
When referring to the purely geometrical data, a shortened form of the full notation is used. To take a common example, 195/55R16 would mean that the width of the tire is 195 mm at the widest point, the height of the side-wall of the tire is 55% of the width (107 mm in this example) & that the tire fits 16 inch diameter wheels.

Less commonly used in the USA & Europe (but often in Japan for example) is a notation that indicates the full tire diameter instead of the side-wall height. To take the same example, a 16 inch wheel would have a diameter of 406 mm. Adding twice the tire height (2x107 mm) makes a total 620 mm tire diameter. Hence, a 195/55R16 tire might alternatively be labelled 195/620R16.

Whilst this is theoretically ambiguous, in practice these two notations may easily be distinguished because the height of the side-wall of an automotive tire is typically much less than the width. Hence when the height is expressed a percentage of the width, it is almost always less than 100% (and certainly less than 200%). Conversely, vehicle tire diameters are always larger than 200 mm. Therefore, if the second number is more than 200, then it is almost certain the Japanese notation is being used - if it's less than 200 then the US/European notation is being used.

Example
The tires on a MINI Cooper might be labelled:

P195/55R16 85H

'P' these tires are for a passenger vehicle.
195 - the width of the tire is 195 mm at the widest point.
55 - indicates that the height of the side-wall of the tire is 55% of the width - 107 mm.
R - this is a radial tire.
16 - this tire fits 16 inch diameter wheels.
85 - the load index, a maximum of 515 kg per wheel in this case.
H - the speed index, this means the maximum permitted speed, here 210 km/h (130 mph).