Aspect ratio is the relationship between the section height and the section width of the tire. This numerical expression is sometimes referred to as series; i.e., 50 Series.

Section Height /Section Width=Aspect Ratio

Example: 235/60R15 91V

Section Width. 235/25.4" = 9.25"

Overall Diameter. 26.10"

26.10 - 15.00" = 11.10"

11.10"/2 = 5.55"

Therefore.

5.55"/9.25= 60%

Tire Dimension Formulas

Section Width

235/25.4 = 9.25"

In this formula 25.4 is a constant number(the numbers of millimeters in one inch), divide the section width in millimeters by 25.4 to determine the section width in inches.

Section Height

Section Width x Aspect Ratio = Section Height

Overall Diameter

(Section Height x 2) + (Wheel Diameter) = Overall Diameter

(5.55" x 2) + 15.00"= 26.10"

Disclaimer:

These possible conversions do not imply interchange-ability. Body and chassis clearance under conditions should be carefully checked for every car model and every time a replacement is made. The overall diameter of the Pirelli replacement tire is included between -3% to +2% of the related O.E.M. tire size. The maximum load index as speed rating of the replacement tire should be equivalent to or higher then the original O.E.M. tire fitment.

Technical data was provided by euro-tire and Pirelli.

Possible Tire Conversions

Rim Size	OE Size	1st Opt.	Plus 1	Plus 2	Plus 3
13	175/70 R13	205/60 R13	185/60 R14	185/55 R15	195/45 R16
	185/70 R13	205/60 R13	195/60 R14	205/50 R15	205/45 R16
14	185/70 R14	195/65 R14	195/60 R15	205/50 R16	215/40 R17
	195/70 R14	215/60 R14	195/65 R15	205/50 R16	215/45 R17

Disclaimer:

These possible conversions do not imply interchangeability. Body and chassis clearance under conditions should be carefully checked for every car model and every time a replacement is made. The overall diameter of the Pirelli replacement tire is included between -3% to +2% of the related O.E.M. tire size. The maximum load index as speed rating of the replacement tire should be equivalent to or higher then the original O.E.M. tire fitment.

Speed Indicator

How Fast Are the Tires Good For?

Speed	Maximum
Index	Speed (in M.P.H.)
K	68
L	74
M	81
N	87
P	93
Q	99
S	112
T	118
H	130
V	149
Z	Over 150

Load Index

What That Means in Carrying Capacity

Load	Weight/	Load	Weight/	Load	Weight/
Index	Tire (#)	Index	Tire (#)	Index	Tire (#)
60	551	80	992	100	1764
61	567	81	1019	101	1819
62	584	82	1047	102	1874
63	600	83	1074	103	1929
64	617	84	1102	104	1984
65	639	85	1135	105	2039
66	661	86	1168	106	2094
67	677	87	1202	107	2138
68	694	88	1235	108	2205
69	716	89	1279	109	2271
70	739	90	1323	110	2337
71	761	91	1356	111	2403
72	783	92	1389	112	2469
73	805	93	1433	113	2535
74	827	94	1477	114	2601
75	853	95	1521	115	2679
76	882	96	1565	116	2756
77	908	97	1609	117	2833
78	937	98	1653	118	2910
79	963	99	1709	119	2998

Uniform Tire Quality Grade Labeling

Toward that objective, we present here an "Official" document that "Describes" the system. We're not sure that this will increase your knowledge of tires, but at least  it explains what some of the markings on them are supposed to convey! Without further ado:

The Uniform Tire Quality Grading System (UTQGS) is a tire information system that
provides buyers with information on three categories:
Each tire manufacturer performs its own tests in these areas, following government prescribed test procedures. Each manufacturer then assigns grades that are branded on the tire. This is known as the Uniform Tire Quality Grade Labeling (UTQGL).

Treadwear

Treadwear grades typically range from 60 to over 500, in twenty point increments. It's important to remember that the actual life of any tire is determined by the road surface quality, driving habits, inflation, wheel alignment and the rotation it experiences. To receive a treadwear grade, a tire is tested under controlled conditions on a government prescribed test course which does not necessarily simulate the actual application for which a given tire is designed to perform. As a result of these test parameters, there is no reliable way to assign miles of wear to treadwear grade points.

Treadwear ratings are determined on a 400 mile government test course covering specified sections of public roads near San Angelo, Texas. A group of not more than four test vehicles travels the course in a convoy so that all tires experience the same conditions. Tread groove depths of the tires being tested are measured after each 800 miles. The same procedure is followed for a set of control or "course monitoring tires." Upon completion of the 7,200 mile test, the rating results of both tests are compared, and the tires being tested are assigned a treadwear rating by the tire manufacturer.

The best way to use treadwear ratings when buying tires is to compare one rating to another. For instance, a tire with a treadwear grade of 400 might be expected to last twice as long as a tire that has a grade of 200.

Traction
 

Traction grades indicate the measurement of a tire's ability to stop a car in straight-ahead motion on a wet test surface pavement. It does not measure straight-ahead acceleration. It's important to remember that traction rating tests are performed only for straight ahead sliding on concrete and asphalt surfaces that have a specified degree of wetting which simulates most road surfaces in a rainstorm. The ratings that result from these tests may not apply to cornering traction or peak values of straight-ahead braking force like those experienced in non-skid braking tests. Traction grades range from A to C, with A being the highest attainable grade.
 

Traction ratings are established on government maintained skid pads. Twenty measurements are taken with an industry standard control tire on an asphalt surface and averaged. The same number of measurements are made on a concrete surface. Corresponding measurements are then made on the tires being tested. Once the results of the tests are compared, traction ratings based on government prescribed coefficient levels are assigned to the tires that were tested.

Temperature

Temperature grades also range from A to C, with A being the highest. Temperature grades represent a properly maintained tire's ability to dissipate heat under controlled indoor test wheel conditions.

Temperature ratings are determined by running tires on an indoor road wheel test under specified conditions. Successive 30-minute runs are made in 5 mph increments starting at 75 mph and continuing until the tire fails. A tire is graded "C" if it meets the minimum performance required by DOT. Grades of "B" and "A" represent higher levels of performance than the minimum required by DOT.

Proper Procedures

Only specially trained persons should mount tires. Improper tire mounting and inflation procedures may cause tire beads to break with explosive force during installation of the tire on the rim causing serious personal injury, death and property damage. Follow the Rubber Manufacturer's Association (RMA) installation and safety procedures for mounting and inflating tires. Tire and rim diameter must match in size. Do not take another person's word for it; check the tire size and the wheel size for proper match. Never inflate tires to fit a wheel mounted by someone else without first checking both wheel and tire size. Clean the rim. Lubricate the rim and beads. If the tire does not seat on the rim at 40 psi, disassemble, recheck sizes, and lubricate before airing up again. Use remote control inflation equipment and inflation cage or some other approved restraining device.