Because most Tyrannitar's are Specially Defensive, and slower than Excadrill, yes Excadrill makes a good counter. a Sash Lead TTar with Counter could ruin you, but those are hella rare.
252+ Atk Mold Breaker Excadrill Earthquake vs. 252 HP / 0 Def Tyranitar: 338-402 (83.6 - 99.5%) -- guaranteed 2HKO
Iron Head does around 80% max damage.
If you're running Excadrill specifically to counter Tyrannitar then I would recommend you run Air Balloon so that Adamant 252 Atk Tyrannitar does not kill you with it's own Earthquake.
