Place meat in a roasting pan. Sprinkle carrots, celery and minced garlic around roast.
In a small bowl, combine the gravy mix with water. Stir in onion soup mix, cream of mushroom soup and cola-flavored carbonated beverage. Pour over the roast.
Cover and cook 1 hour in the preheated oven.
Reduce oven temperature to 225 degrees F and continue cooking 2 hours.
Remove from oven, and turn roast over so that the top is now covered with the gravy. Cover pan, and return to oven for a minimum of 2 hours.
Remove from oven and let meat rest for 10 minutes before slicing.