Excellent tire technology and terminology links available here:

Tire Tech (Tirerack.com)

Winter Tire Tech

The first link contains info on the following:

General Tire Info
Air Pressure
Air Pressure vs. Dry Performance
Air Pressure vs. Wet Performance
Air Pressure, Temperature Fluctuations
Air Pressure, Time Fluctuations
Air Pressure/Load Adjustment for High Speed Driving
Alignment
Breaking In Your Tires
Bridgestone's UNI-T Technology
Calculating Tire Dimensions
Checking Tire Inflation Pressure
Construction Materials
Contact Patch
Determining the Age of a Tire
Diagnosing Tire Pull
Driving Through Tire Blowouts
Economic Commission for Europe (E.C.E.)
Firestone's UNI-T Technology
How Do I Compare Price vs. Value?
Hydroplaning: The Role Tires Play
Match Mounting to Enhance Tire & Wheel Uniformity
Matching Tires on Four-Wheel Drive And All-Wheel Drive Vehicles
Michelin's C3M Process
Mounting and Balancing
North American Load and Pressure Markings
Original Equipment (OE) Tires
Original Equipment Tire Designations
P-Metric and Euro Metric Tire Sizing
Porsche N-Specification Tire Approvals
Puncture Repairs
Puncture Repairs the Right Way
Rolling Resistance
Rubber Cracking
Selecting the Right Tires
Sidewall Indentations, Undulations and Protrusions
Sidewall Markings
Specific Mileage Warranties
Speed Rating, Load Index & Service Descriptions
Speedometer Accuracy



Storing Tires
The Plus Concept
Tire & Wheel Owner's Manual
Tire & Wheel Package Installation Instructions
Tire & Wheel Package Ride Uniformity Confirmation
Tire Aging
Tire Flatspotting
Tire Pressure Monitoring Systems
Tire Rotation
Tire Size Conversion Chart
Tire Size Information
Tramlining: Coping with the Ruts in the Road
Uniform Tire Quality Grade (UTQG) Standards
Vibration Diagnosis Flow Chart
Warranty
When Should I Replace My Tires?
Where to Install New Pairs of Tires?


Light Truck Tires
Diameter Comparison of Light Truck Tire Sizes
Dual Tires for Light Truck Use
Light Truck Fitment Formula
Load Range/Ply Rating Identification
Load Reduction of Euro- and P-Metric Tires on Light Trucks


Run Flat Tires
Run Flat Tires
Testing the Dunlop SP Sport 9000 DSST


Competition Tires
Air Pressure for Competition Tires
Care and Feeding of the Kumho ECSTA V700
Care and Feeding of the Kumho V700 Victoracer
Care and Feeding of the Michelin Pilot Sport Cup
Care and Feeding of the Pirelli P Zero Corsa
Getting More Out of Competition Tires
Hoosier Racing Tire Tips
Hoosier Wet Radial Installation
Our Heat Cycling Service
Shaving Tires for Competition

The second link contains winter tire info on these subjects:

Breaking-In New Winter Tires
Bridgestone Blizzak FAQs
Four Winter Tires...The Only Way To Go
How to Confirm a Winter Performer
Removing Studs from Winter Tires
Size Selection of Winter Tires (Base Model and Minus Sizing)
Tire Chains for Winter Use

Traction, Confidence and Control
We Know How They Go in Ice and Snow
Why Electronic Drivers Aids and 4wd Systems Aren't Enough
Winter Driving Tips
Winter Maintenance Tips
Winter Tire FAQs
Winter Tires for SUVs, Pickups and Minivans

__________________
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No, I'm not Bob Lutz...
Link to Solstice Technical FAQ
Link to Solsticeforum Rules/Guidelines/Etiquette
Link to Solsticeforum Site Index (under const.)
Just go buy the car you fell in love with!

Using a P195/60R15 87S tire size as our example, the 87S at the end of the size represents the tire's service description. A service description identifies the tire's load index and speed rating. Service Descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991.

The first two digits (87S) represent the tire's load index and are followed by a single letter (87S) identifying the tire's speed rating.

Load Index

P195/60R15 87S - The load index (87) is the tire size's assigned numerical value used to compare relative load carrying capabilities. In the case of our example the 87 identifies the tires ability to carry approximately 1,201 pounds.

The higher the tire's load index number, the greater its load carrying capacity.

89 = 1,279 pounds
88 = 1,235 pounds
87 = 1,201 pounds
86 = 1,168 pounds
85 = 1,135 pounds

A tire with a higher load index than that of the Original Equipment tire indicates an increase in load capacity. A tire with a load index equal to that of the Original Equipment tire indicates an equivalent load capacity. A tire with a lower load index than the Original Equipment tire indicates the tire does not equal the load capacity of the original.

Typically, the load indexes of the tires used on passenger cars and light trucks range from 70 to 110.

Load Index Pounds Kilograms -- Load Index Pounds Kilograms

71 761 345 -- 91 1356 615
72 783 355 -- 92 1389 630
73 805 365 -- 93 1433 650
74 827 375 -- 94 1477 670
75 853 387 -- 95 1521 690
76 882 400 -- 96 1565 710
77 908 412 -- 97 1609 730
78 937 425 -- 98 1653 750
79 963 437 -- 99 1709 775
80 992 450 -- 100 1764 800
81 1019 462 -- 101 1819 825
82 1047 475 -- 102 1874 850
83 1074 487 -- 103 1929 875
84 1102 500 -- 104 1984 900
85 1135 515 -- 105 2039 925
86 1168 530 -- 106 2094 950
87 1201 545 -- 107 2149 975
88 1235 560 -- 108 2205 1000
89 1279 580 -- 109 2271 1030
90 1323 600 -- 110 2337 1060

Speed Rating

In Europe, where selected highways do not have speed limits and high speed driving is permitted, speed ratings were established to match the speed capability of tires with the top speed capability of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at "unusual" mile per hour increments). Despite the tire manufacturer's ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits.

Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met.

It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer's original speed rating, even after being repaired because the tire manufacturer can't control the quality of the repair.

Over the years, tire speed rating symbols have been marked on tires in any of three ways shown in the following examples:

225/50SR16 225/50SR16 89S or 225/50R16 89S

Each of these was an acceptable method of identifying speed ratings.

Early tires had their speed rating symbol shown "within" the tire size, such as 225/50SR16. Tires using this type of branding were not to have been produced after 1991.

225/50SR16 112 mph, 180 km/h
225/50HR16 130, 210 km/h
225/50VR16 in excess of 130 mph, 210 km/h

Beginning in 1991, the speed symbol denoting a fixed maximum speed capability of new tires must be shown only in the speed rating portion of the tire's service description, such as 225/50R16 89S. The most common tire speed rating symbols, maximum speeds and typical applications are shown below:

M 81 mph 130 km/h
N 87 mph 140km/h Temporary Spare Tires
P 93 mph 150 km/h
Q 99 mph 160 km/h Studless & Studdable Winter Tires
R 106 mph 170 km/h H.D. Light Truck Tires
S 112 mph 180 km/h Family Sedans & Vans
T 118 mph 190 km/h Family Sedans & Vans
U 124 mph 200 km/h
H 130 mph 210 km/h Sport Sedans & Coupes
V 149 mph 240 km/h Sport Sedans, Coupes & Sports Cars

When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of new vehicles that have extremely high top-speed capabilities.

W 168 mph 270 km/h Exotic Sports Cars
Y 186 mph 300 km/h Exotic Sports Cars

While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire's 168 mph, 270 km/h maximum speed.

225/50ZR16 in excess of 149 mph, 240 km/h
205/45ZR17 88W 168 mph, 270 km/h
285/35ZR19 99Y 186 mph, 300 km/h

Most recently, when the Y-speed rating indicated in a service description is enclosed in parentheses, such as 285/35ZR19 (99Y), the top speed of the tire has been tested in excess of 186 mph, 300 km/h indicated by the service description as shown below:

285/35ZR19 99Y 186 mph, 300 km/h
285/35ZR19 (99Y) in excess of 186 mph, 300 km/h

As vehicles have increased their top speeds into Autobahn-only ranges, the tire speed ratings have evolved to better identify the tires capability, allowing drivers to match the speed of their tires with the top speed of their vehicle.

__________________
Magister Solstitium Sum!!!
No, I'm not Bob Lutz...
Link to Solstice Technical FAQ
Link to Solsticeforum Rules/Guidelines/Etiquette
Link to Solsticeforum Site Index (under const.)
Just go buy the car you fell in love with!