Assuming max IVs (31) for both, 248 speed evs are necessary for unboosted Hydreigon to always outspeed unboosted Haxorus.
However, if both have max speed evs (252) and the Haxorus has 2 speed IVs more than Hydreigon, they speed tie.
If the Haxorus has 3 or more speed IVs than Hydreigon and both have max speed evs, Haxorus will always outspeed Hydreigon. --- Creedy: Die! Die! Why won't you die? V: Beneath this mask there is more than flesh. Beneath this mask there is an idea, Mr. Creedy, and ideas are bulletproof.