In a small bowl, mix together chili powder, cumin, garlic powder, onion powder, salt, and pepper.
Rub the spice mixture all over the beef chuck roast.
Place the seasoned roast in the slow cooker.
Cover and cook on low for 6-8 hours, or until the beef is tender and easily shreds.
Remove the beef from the slow cooker and place it on a cutting board.
Using two forks, shred the beef.
Return the shredded beef to the slow cooker with the cooking liquid and stir to combine. Let it simmer for another 15-30 minutes to absorb the flavors.
Warm the tortillas according to package directions.
Fill tortillas with shredded beef and top with desired toppings such as cilantro, onion, salsa, and sour cream. Serve with lime wedges.
Notes
For a spicier taco, add a pinch of cayenne pepper to the spice rub.