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To understand the real world conditions of use of plug-in hybrid electric vehicles, the electrical energy consumption and gasoline energy consumption must be weighted according to how often a consumer will drive fueled by each energy source. To perform this weighting The Society of Automotive Engineers Hybrid Committee has codified the concept of the utility factor in SAE J2841 - Utility Factor Definitions for Plug-In Hybrid Electric Vehicles Using Travel Survey Data. The J2841 utility factor weights the energy consumption from each energy source according to a model of US driving behavior derived statistics from the 2001 National Highway Transportation Survey, and the assumption that each vehicle begins the day trip fully charged and does not charge over the course of the day. This paper examines the sensitivity of the J2841 utility factor to more detailed models of vehicle charging behavior and proposes a utility factor model and simulation method as an alternative to J2841.