Educational Center – Common Terms and Glossary

How to Read Tire Size

Reading a tire size involves decoding an alphanumeric string stamped on the tire sidewall. Sidewall of your tire is like a passport, providing crucial details about its dimensions, type, and performance. Understanding these markings is essential for finding the right tires for your vehicle and driving requirements.

Example: P225/70R16 91S

• P: Tire Type – This indicates the tire is a P-Metric tire, designed for passenger vehicles. LT means light truck, ST means Special Trailer, and T means a temporary spare.
• 225: Tire Width – The width of the tire in millimeters, measured from sidewall to sidewall. In this case, the tire is 225 millimeters wide.
• 70: Aspect Ratio – This number represents the tire's height (from the rim to the tread) as a percentage of its width. Here, the height is 70% of the 225 mm width.
• R: Construction – Stands for radial construction, the most common type for modern vehicles.
• 16: Rim Diameter - This is the diameter of the wheel in inches that the tire is designed to fit.
• 91: Load Index – This assigned code indicates the maximum weight a tire can carry at its specified pressure.
• S: Speed Rating – A letter indicating the maximum speed a tire is designed to handle safely under specific conditions.

Tire Type

Common tire type prefixes:

• P – Passenger car.
• LT – Light truck.
• ST – Special trailer.
• C – Commercial van tire.
• XL / HL / Reinforced – Tires designed to carry higher loads than standard tires of the same size.
• T – Temporary spare tire.

Tire Construction Types

The three main types of tire construction are: radial, bias-ply (cross-ply), and belted bias.

Radial Tires: The body plies run straight across, from bead to bead, at a 90-degree angle to the tread centerline, perpendicular to the direction of travel, with stabilizing steel or fabric belts under the tread. These tires are standard on almost all modern passenger cars, trucks, and SUVs. Radial tires feature lower rolling resistance, better fuel economy, longer tread life, and higher high-speed stability.

Bias-Ply (Cross-Ply) Tires: The cords in the body plies run in an overlapping criss-cross pattern (usually at an angle of 30 to 45 degrees) from bead to bead. These tires are used in vintage vehicles, heavy-duty off-road equipment, and agricultural or industrial applications. They feature thick, rigid sidewalls, providing strong resistance to punctures. However, bias-ply tires have a higher rolling resistance and wear out faster at highway speeds.

Belted Bias Tires: These tires have a traditional bias-ply carcass with added stabilizing steel or fiber belts directly underneath the tread. Belted bias tires are less efficient compared to standard radial tires, but they offer a stiffer tread area for better high-speed performance compared to regular bias-ply tires. For this reason, these tires were commonly used in older classic muscle cars.

Tire Width

The Width of a tire, measured in millimeters, directly impacts its contact patch with the road. A wider tire typically offers improved grip, better handling, and a more visually appealing appearance. The more rubber in contact with the road, the better the grip, especially in dry conditions and during acceleration or braking. Wider tires can enhance cornering stability and responsiveness, but they may also increase rolling resistance and slightly reduce fuel efficiency.

Aspect Ratio

The Aspect Ratio represents the sidewall height as a percentage of the tire's width. A 225 mm width tire with a higher aspect ratio (e.g. 70) will have a taller sidewall, while a lower aspect ratio (e.g. 45), will have shorter sidewall. This value helps ensure proper fit of the wheel and adequate clearance in the vehicle's wheel well.

Load Index

The Load Index is a numerical representation found on the tire sidewall, indicating the maximum load that each tire is designed to support when inflated to its recommended pressure. Always opt for tires with a load index that matches or surpasses the manufacturer's guidelines for your vehicle.

Load index chart

Load index	Max Weight (kg)	Max Weight (lbs)
51	195	430
52	200	441
53	206	454
54	212	467
55	218	481
56	224	494
57	230	507
58	236	520
59	243	536
60	250	551
61	257	567
62	265	584
63	272	600
64	280	617
65	229	639
66	300	661
67	307	677
68	315	694
69	325	717
70	335	739
71	345	761
72	355	783
73	365	805
74	375	827
75	387	853
76	400	882
77	412	908
78	425	937
79	437	963
80	450	992
81	462	1019
82	475	1047
83	487	1074
84	500	1102
85	515	1135
86	530	1168
87	545	1202
88	560	1235
89	580	1279
90	600	1323
91	615	1356
92	630	1389
93	650	1433
94	670	1477
95	690	1521
96	710	1565
97	730	1609
98	750	1653
99	775	1709
100	800	1764
101	825	1819
102	850	1874
103	875	1929
104	900	1984
105	925	2039
106	950	2094
107	975	2150
108	1000	2205
109	1030	2271
110	1060	2337
111	1090	2403
112	1120	2469
113	1150	2535
114	1180	2601
115	1215	2679
116	1250	2756
117	1285	2833
118	1320	2910
119	1360	2998
120	1400	3086
121	1450	3197
122	1500	3307
123	1550	3417
124	1600	3527
125	1650	3638
126	1700	3748
127	1750	3858
128	1800	3968
129	1850	4079
130	1900	4189
131	1950	4299
132	2000	4409
133	2060	4542
134	2120	4674
135	2180	4806
136	2240	4938
137	2300	5071
138	2360	5203
139	2430	5357
140	2500	5512
141	2575	5677
142	2650	5842
143	2725	6007
144	2800	6173
145	2900	6393
146	3000	6614
147	3075	6779
148	3150	6945
149	3250	7165
150	3350	7385

Speed Rating

The Speed Rating is an alphabetical code on a tire, indicating the maximum speed a tire is designed to handle safely under specific conditions.

Speed rating chart for passenger cars

Speed Symbol	Speed (kmph)	Speed (mph)
E	70	43
F	80	50
G	90	56
J	100	62
K	110	68
L	120	75
M	130	81
N	140	87
P	150	94
Q	160	100
R	170	106
S	180	112
T	190	118
U	200	124
H	210	130
V	240	149
W (ZR*)	270	168
Y (ZR*)	300	186

* Tires capable of speeds over 240 km/h (149 mph) may include "ZR" in their size designation. For those with speeds over 300 km/h (186 mph), "ZR" is mandatory.

Tire Sizing Systems

Different regions and vehicle types use different tire sizing systems. It is important to understand these differences in order to choose the correct tires for your vehicle. Let's learn about the most common tire sizing systems.

P-Metric Sizing System

The P-Metric system, introduced in 1976, is widely used for passenger vehicles in North America. It combines metric measurements, such as tire width in millimeters, with imperial units, such as rim diameter in inches. P-Metric tires are designed to have lower rolling resistance and higher inflation pressure compared to older tire sizing systems.

Example: P215/70R16

ISO Metric Sizing System

The ISO (International Organization for Standardization) Metric system is used globally and combines metric measurements with a service description that includes the load index and speed rating.

Example: 215/65R16 102T

LT-Metric Sizing System

The LT-Metric tire system is similar to the P-Metric system, but is specifically designed for light trucks. Instead of the "P" designation, the LT designation indicates that the tire can handle higher loads and pressures typically associated with trucks.

Example: LT215/75R15

Light Truck Numeric Sizing System

The Light Truck Numeric system employs a combination of numbers and alphabetic characters to represent tire size, similar to the P-Metric system.

Example: 31x10.50 R16 LT