Bring a large pot of lightly salted water to a boil. Add the macaroni, and cook until tender, about 8 minutes. Rinse under cold water and drain.
In a large bowl, mix together the mayonnaise, vinegar, sugar, mustards, garlic powder, salt and pepper.Stir in the onion, celery, green pepper, carrot, pimentos and macaroni. Refrigerate for at least 4 hours before serving, but preferably overnight.