It makes it more flexible and supple (and thinner) because there's less rubber in the carcass, which is the result of thinner threads, not because of them.



A larger thread equates to a lower tpi and a stiffer casing, notes former professional cyclist Ian Dille writing in Mountain Bike magazine. A standard downhill mountain bike tire might use a 60 tpi casing, while a performance cross-country tire or road bike tire might be 120 tpi. A smaller thread size, he notes, makes for higher tpi and less rubber packed between the threads, making the tire lighter and more supple on the road. Cross-country tires have a single sheet of casing bonded under the center of the tread, Dille writes, while all-mountain tires use a two-ply casing.

- See more at: What Is the TPI on Bicycle Tires? | LIVESTRONG.COM