It's time to revisit a subject which Ross introduced some years ago. There's a wealth of information written on either side of your automotive or motorcycle tires, and it pays to be able to read it.
This is a simple discussion about a complex subject, so feel free to add your comments regarding other information this skips over. Everyone can benefit from what you know.

Let's work with this hypothetical tire size and explain what each part of the order number means:
P185 65 R 15 84 H 5195

*P stands for "passenger", as opposed to a truck or off-road tire, etc

*185 represents the width of the tread, in millimeters (or 7.2") wide

*65 is the "aspect ratio", measuring the distance btwn the wheel rim and the
outside circumference of the tire (the "height" of the tire). The number is
expressed as the PERCENTAGE of the tire width in millimeters; the height of this tire is "65"% of the width of the tread (or 120mm, about 4.7" tall on either side of the rim. Multiply the aspect ratio times two and add the rim diameter to determine the overall height of your intended wheel/tire combination. Convert to either inches or millimeters; you can't add them together)

*R designates it's a Radial tire, not a bias ply or other construction

*15 is the diameter size of the steel rim, in INCHES

*84 is the Load Index, telling how much weight the tire can withstand. 84 is
1,102 pounds

*H this is the speed rating; the maximum speed the tire is designed to sustain for 10 minutes under ideal conditions without failing. H represents 130mph. (A list below shows other speeds)

*51/95 The week and year of manufacture. Our hypothetical tire was made in late
December 1995.

Here is a list of the "speed code" designations on passenger tires:
G 56mph (spare tires)
J 62mph
K 68mph
L 75mph
M 81mph
N 87mph
(no "O")
P 93mph
Q 99mph
R 106mph
S 112mph
T 118mph
U 124mph
H 130mph
V 149mph
W
Z
Y 186mph
(Y) 186+mph
Here's Tire Rack's useful TIRE LOAD INDEX CHART for everything from mopeds to earth movers. The parenthetical numbers (0 thru 150) are the designation found on the tire, followed by the weight it can withstand in pounds. As you can see, our hypothetical tire has an 84 rating, good for 1102 pounds:

Load- Carrying Capacity Index Ratings
Load Index Load (lbs)
(0) 99 (50) 419 (100) 1764
(1) 102 (51) 430 (101) 1819
(2) 105 (52) 441 (102) 1874
(3) 107 (53) 454 (103) 1929
4) 110 54) 467 104) 1984
5) 114 55) 481 105) 2039
6) 117 56) 494 106) 2094
7) 120 57) 507 107) 2149
8 ) 123 58) 520 108) 2205
9) 128 59) 536 109) 2271
10) 132 60) 551 110) 2337
11) 136 61) 567 111) 2403
12) 139 62) 584 112) 2469
13) 143 63) 600 113) 2535
14) 148 64) 617 114) 2601
15) 152 65) 639 115) 2679
16) 157 66) 639 116) 2756
17) 161 67) 677 117) 2833
18) 165 68) 694 118) 2910
19) 171 69) 716 119) 2998
20) 176 70) 739 120) 3086
21) 182 71) 761 121) 3197
22) 187 72) 783 122) 3307
23) 193 73) 805 123) 3417
24) 198 74) 827 124) 3527
25) 204 75) 852 125) 3638
26) 209 76) 882 126) 3748
27) 215 77) 908 127) 3858
28) 220 78) 937 128) 3968
29) 227 79) 963 129) 4079
30) 234 80) 992 130) 4189
31) 240 81) 1019 131) 4289
32) 247 82) 1047 132) 4409
33) 254 83) 1074 133) 4541
34) 260 84) 1102 134) 4674
35) 267 85) 1135 135) 4806
36) 276 86) 1168 136) 4938
37) 282 87) 1201 137) 5071
38) 291 88) 1235 138) 5203
39) 300 89) 1279 139) 5357
40) 309 90) 1323 140) 5512
41) 320 91) 1356 141) 5677
42) 331 92) 1389 142) 5842
43) 342 93) 1433 143) 6008
44) 353 94) 1477 144) 6173
45) 364 95) 1521 145) 6393
46) 375 96) 1565 146) 6614
47) 386 97) 1609 147) 6779
48) 397 98) 1653 148) 6844
49) 408 99) 1709 149) 7165
150) 7385


I know you didn't ask for this, but you could have wasted your 7 minutes in some non-productive way, so this gives you a new thing you learned today.

This really isn't useless trivia. Knowing this stuff can help you mount the best tire for your intended driving, or prevent you from falling for a "good deal" on a tire made too long ago to still be safe.

But most important is that you will not buy tires which your car can "outrun". Shredding a tire at over 100mph will quickly change your mood and spoil all your fun.

Your comments and observations are welcome.
Frank T

_________________
1970 240Z

Great information. I knew some of that stuff but there was quite a bit I didn't. The date code for example, I just thought those numbers were there to be ignored

After 30 years as a state police officer, my grandfather always said the most important part of a car was the tires, your car's only contact point with the ground. He did an awful lot of driving in his day, and investigated many vehicle accidents. I am sure that conclusion was drawn from that experience.

Jay