An article in today's New York Times
argues that your garage door is "a potentially large source of heat loss" that should make you consider replacing your old door with a new, insulated model. The article is very unpersuasive. Most garages are not heated, so adding an insulated door, or even insulating the walls, isn't going to have much effect on heat loss from the house.
Now, there are a lot of good reasons for replacing a garage door, just as there are a lot of good reasons for putting a heater in a garage that is used regularly as a workshop or for some other purpose. But insulation works best when it is located between heated and unheated spaces. Insulating an unheated garage strikes me as a silly waste of money.