Tire Challenge Gravel Grinder VTLR 700x38 Black/BrownChallenge Gravel Grinder Race Tire 700 x 38 Tubeless Folding Black Brown. Intended for all around or dirt gravel roads this tire comes equipped with a file tread center and a knobbier shoulder tread. File tread center pattern offers low rolling resistance on smoother terrain and increased grip when the road gets rough Shoulder tread is more aggressive for corner confidence and additional control The Gravel Grinder is ideal for hardpack gravel sand