Understanding the fuel economy of vehicles in actual use has important implications for fuel economy, greenhouse gas emission and consumer information policies. This study explores how fuel economy varies with intensity of daily vehicle use, cumulative mileage, and ambient temperature. Using a unique longitudinal database, we quantify variations in fuel economy (miles per gallon or MPG) over time for the same vehicle. The database consists of more than 600,000 odometer and fuel purchase records, obtained from the “My MPG” section of the http://fueleconomy.gov website. Over 10,000 drivers reported their fuel purchases and vehicle usage, with an average of 36 fill-ups per vehicle. Multilevel models are used to analyze and compare relationships for conventional gasoline and hybrid vehicles. For gasoline vehicles, within-vehicle variation accounts for 23% total variation of fuel economy and 77% is between-vehicle variation. For hybrids, the equivalent proportions are 19% and 81%. On-road fuel economy increases nonlinearly with cumulative mileage, with nearly all of the increase occurring within the first few thousand miles. The estimated trend for hybrid vehicles is very different from that of conventional gasoline vehicles. Hybrids were found to have higher daily miles of use than conventional gasoline vehicles. Cold temperatures appear to have a greater effect on the fuel economy of hybrids than conventional gasoline vehicles.