Abstract Although the product-centered focus of life cycle analysis has been one of its strengths, this analytical perspective embeds assumptions that may conflict with the realities of the problems to which it is applied. This paper demonstrates, through a series of mathematical derivations, that all the products in use, rather than a single product, frequently should be the appropriate unit of analysis. Such a "fleet-centered" approach supplies a richer perspective on the comparative emissions burdens generated by alternative products, as well as eliminating certain simplifying assumptions imposed upon the analyst by a product-centered approach. A sample numerical case, examining the comparative emissions of steel-intensive and aluminum intensive automobiles, is presented to contrast the results of the two approaches. The fleet-centered analysis shows that the "crossover time" (i.e., the time required before the fuel economy benefits of the lighter aluminum vehicle offset the energy-intensity of the processes used to manufacture the aluminum in the first place) can be dramatically longer than that predicted by conventional life cycle analyses. The fleet-centered perspective explicitly introduces the notion of time as a critical element of comparative life cycle analyses and raises important questions about the role of the analyst in selecting the appropriate time horizon for analysis. Moreover, with the introduction of time as an appropriate dimension to life cycle analysis, the influences of effects distributed over time can be more naturally and consistently treated.