Place meat, carrots, celery and garlic in slow cooker.
Combine gravy mix and 2 tbsp. water. Add onion soup mix, cream of mushroom soup, and cola. Pour over roast.
Cook 2-3 hours in slow cooker on low.
Turn roast over so that the top is now covered with gravy, and cook another 2-3 hours.
Remove meat, and let rest at least 10 minutes before slicing it.