A normal Garchomp should have atleast 240 EVs in speed.
Why you ask? So that it always outspeed Mega Charizard variants (before Dragon Dance).
With 240 EVs, it's base speed hits 330 at level 100, while Charizard hits 328 with max speed.
But, if both Pokemon are level 50, 244 EVs should be used instead, as then it hits 168 Speed and MegaZard gets outsped by 1 speed point (167 at max).
EDIT: This was with Jolly Nature. Adamant Garchomp fails to outspeed many Pokemon altogether, including all base 100 speed.
Hope I helped!