Determining the Size of Tyres on Your Car

Sidewall Markings

Look at the sidewall of your tyre and you'll notice different markings and codes. One of them represents your tyre's size.

Tyre Sizes

Various numbers and letters are joined together to show the tyre size. For example - 205/55 R16 82 H.
What does it mean?

205 - Width of the tyre in milimetres.
55 - The ratio of the tyre's height to the tyre's width, expressed in percentages, popularly known as the aspect ratio.
R - Tyre's construction type - in this instance, RADIAL. If your tyre doesn't have an "R" but rather "A", "B", or "X", we're talking about bias ply construction.
16 - Rim diameter in inches.
82 - Load Index, or how much weight can the tyre handle.
H - Speed Rating, or how fast is too fast for the tyre.

This is all the basic information for an educated tyre purchase. If you want to delve deeper into the "writings on the wall", do refer to the information below.

Aspect Ratio

Aspect ratio, or profile helps us determine what percentage of tyre's width goes into the height of its sidewall. The bigger the percentage, the bigger the driving comfort but at the expense of handling properties. It's usual for buses and trucks to have high aspect ratios (up to 95% of tyre's width!). Sports vehicles, on the other hand, have extremely low aspect ratios. You must have heard about the low-profile tyres before. Their sidewalls are rather short (up to 30% of tyre's width), which brings about sport-grade handling at the expense of driving comfort. Much of the vibrations gets passed on to the suspension. One of the reason race cars have upgraded suspension systems.

Speed Ratings

Every tyre has its speed limit, depicted by a certain letter. For the most part, the higher up the alphabet scale the letter is, the faster your tyre can run, with two exceptions to the rule. Consult the table to see what we mean.

Speed symbol	Max speed mph	Max. speed km/h
N	87	140
P	93	150
Q	99	160
R	106	170
S	112	180
T	118	190
H	130	210
V(and ZR*)	149	240
W	168	270
Y	186	300

*ZR marking was used before 1990s to mark the high-performace tyres.

Load Index

The last two-digit number denotes the tyre's maximum load capacity when reaching its speed rating limit.

Load Index	kg
64	280
65	290
66	300
67	307
68	315
69	325
70	335
71	345
72	355
73	365
74	375
75	387
76	400
77	412
78	425
79	437
80	450
81	462
82	475
83	487
84	500
85	515
86	530
87	545
88	560
89	580
90	600
91	615

Load Index	kg
92	630
93	650
94	670
95	690
96	710
97	730
98	750
99	775
100	800
101	825
102	850
103	875
104	900
105	925
106	950
107	975
108	1000
109	1030
110	1060
111	1100
112	1120
113	1150
114	1180
115	1215
116	1250
117	1285
118	1320
119	1360

DOT code

DOT is the abbreviation for Department of Transportation and it's comprised of the tyre factory code and the production date. The factory codes are too cumbersome to explain in this post and aren't as important as the production date code.

Tyre Age

If you're asking yourself "how old are my tyres?", look for either a three-digit number or a four-digit number on the tyre sidewall. Three-digit numbers represent tyres made before 2000, four-digit ones the tyres made after 2000. If you're on the south side of 2000, you definitely need a tyre change. And not only then. Generally, if the tyre is older than ten years, it should be replaced, even if the tread depth is within bounds. Production year of your tyre is represented by the last two digits. The first two digits represent the production week.

Other Tyre Markings

M+S - Stands for tyres capable of helping you drive through mud and snow.
Three-Peaked Mountain and Snowflake Symbol (Alpine Symbol) - Stands for tyres built to withstand and trump the horrors of sub-zero temperatures and icy roads.
E-Mark - Stands as a proof that the tyre is within EU or International (UNECE) standards regarding its dimensions, load index, speed rating, as well as having an adequate tread depth.