Mountain bikes tires don’t just come in a variety of shapes and sizes. They also come in different treads, casings, and rubber compounds. These various combinations are used to create tires for specific trail conditions and riding styles. Specialized has developed their own rating system for tread compounds, similar to the Maxxis MaxxTerra, MaxxGrip, and MaxxRace.
Tire compounds are important because they play a major part in the feel of your bike. Tire engineers combine hundreds of ingredients to create different compounds with precise hardness, elasticity, durability, and rebound. At Specialized, these compounds are separated into different T ratings for the level varying levels of damping and grip.
T5
The T5 tread compound is highly durable, has low rolling resistance, and is a great option for cross country racing. Tires that have the T5 compound include the Renegade and Fast Trak which come stock on bikes like the Epic and Chisel. If speed is the priority, look for the T5 compound.
T7
T7 is the standard trail tread compound for Specialized. It has low rebound and low hardness which makes for a grippy and supple feel while maintaining durability. This tread compound can be found on popular models like the Eliminator and Purgatory and is a great option for trail and enduro style riding. A T7 used in the rear with a T9 in the front makes for a great combination, too!
T9
Specialized developed the T9 tread compound for those riding the gnarliest terrain, where grip is essential. This compound can be found on the Hillbilly and Butcher. While it excels in the grip category, it is lacking in durability and speed. Rocky terrain will wear this tread out quickly, so we recommend this for tread compound for loamy and softer trails.
T5/T7/T9 Combinations
Certain tires like the Fast Trak and Eliminator come with a combination of tread compounds like T5/T7 and T7/T9. These tires use a faster rolling compound center tread and grippier compound shoulder lugs, for a combination of speed, durability, and cornering traction.
Peak Cycles - BikeParts.com Blog 