In a zip-top plastic bag combine 1/4 cup oil, minced garlic, minced rosemary, minced thyme, 1 tablespoon salt, and 1½ teaspoons freshly ground black pepper.
Pat the 2½-pound tri-tip dry with paper towels and score the fat layer with a sharp knife (cut through the fat but not into the meat).
Add the tri-tip to the bag and turn to coat thoroughly with the marinade; refrigerate at least 1 hour and up to 2 hours.
Remove the tri-tip from the refrigerator about 30 minutes before grilling to bring it to room temperature.
Preheat the grill on high for 10 minutes, then clean and oil the grate.
Sear the tri-tip over direct high heat, turning occasionally, about 3–4 minutes per side, until browned and slightly charred.
Move the tri-tip to indirect heat and continue cooking 20–30 minutes more until the internal temperature reaches your desired doneness: 120–125°F rare, 130–135°F medium-rare, 140–145°F medium, 150°F medium-well, or 160°F well-done.
Transfer the tri-tip to a cutting board and let it rest 10–15 minutes.
Slice the tri-tip thinly against the grain and serve.