In-use fuel economy shortfall varies regionally and seasonally in response to changing environmental conditions and travel patterns. Shortfall changes by 25 percent or more are associated with variations in miles travelled per day by in-use vehicles. Monthly variations in northern states are twice those in the south; air conditioning penalties also vary by region. Although regional differences in annual levels are relatively small, shortfall can increase by 16 percent in densely populated urban areas. These variations have been quantified in an analysis of time-series mpg data and related to variables commonly reported in fuel economy surveys.