Place roast in a large bowl. Mix remaining ingredients in small bowl and pour over roast.
Marinate in fridge for 30 minutes. Turn over and marinate for additional 30 minutes.
Preheat oven to 350 and place roast in a roasting dish with a lid. Cover and bake for 2 hours. Reduce heat to 250, turn roast over, and continue cooking for 1 hour, covered.
Remove roast from oven and transfer to serving platter. Squeeze juice from 1 section of grapefruit over roast. Allow to settle for 10 minutes before cutting. Garnish with remaining sections of grapefruit.