Although distinction between "renewable" and "non- renewable" energy resources has important political ramifications, in reality all practical energy production chains use a combination of resources that are renewable to different degrees. Current approaches to definition of renewability are overly simplistic and fail to account for weak formulations of sustainability that allow for substitution of resources. This research presents a mathematical model for assessing the renewability of corn ethanol-gasoline blends in terms of the number of years of production required to replace consumed resources with acceptable substitutes. Weak versus strong representations of sustainability are compared by variation of a substitutability parameter. The principal conclusion of this study is that ethanol blends are more rapidly renewable than petroleum-based gasoline, but that increasing the percentage of ethanol in the blend beyond about E20 results in only marginal improvements in renewability.