Originally Posted by chaadster

TPI is Threads Per Inch, and refers to the tire casing. It matters because higher TPI casings are more supple, which allows for a lighter tire that feels better (i.e. smoother) and rolls more efficiently, the reason being that the more supple, flexible, casing deforms around road irregularities more readily, both maintaining more forward momentum rather than being bounced off on another vector and being transmitted into the wheel, and ultimately, into you. I also think higher TPI allows for higher PSI, which for heavier riders especially, can be crucial.

On the flip side, lower TPI tires, because they're heavier, tend to be more durable and cut resistant (esp. at the sidewalls), and are probably a little better suited to running at lower PSI for those reasons.

Practically, however, the "feel" issue is huge for me, and I think that no serious roadie concerned with performance could go from, say, a 127tpi tire back to a 27tpi one without feeling really cheated. Quality tires are well worth it, IMO.