As a garbage truck ages, its maintenance and repair costs increase, and its availability for service decreases. A large city's decision support system tracks this decline pattern for each truck and specifies a replacement age for it. But that system is shown here to be based upon a flawed formula which will, inevitably, keep any poorer than average truck too long and replace any better than average truck too soon. An alternative, economically—sound base is proposed for this and similar systems. Methodology for computing the excess cost caused by using the present system's non—optimal recommendations is developed and illustrated.