Preheat oven to 425 °F. Grate and juice 1 lemon (1 tbsp each). Mix peel, juice, garlic, and pepper; reserve 1 tbsp for potatoes. Rub remaining mixture over beef tri-tip roast. Cut remaining lemon into 12 wedges; set aside.
Place roast on rack in shallow pan; do not cover or add water.
On a rimmed sheet pan, toss potatoes with reserved lemon mixture, lemon wedges, oil, and 1/2 tsp salt.
Roast beef 30 to 40 minutes for medium rare, 40 to 50 minutes for medium. Roast potatoes 25 to 30 minutes or until tender, stirring once.
Remove roast when thermometer reads 135 °F (medium rare) or 150 °F (medium). Tent with foil; rest 15 to 20 minutes (temp will rise to 145 °F or 160 °F, respectively).
Slice beef across the grain; season with salt. Serve with potatoes and lemon wedges.