This is an excerpt from EERE Network News, a weekly electronic newsletter.
Arizona Fleets Report Unexpectedly Low Mileages for Hybrid Vehicles
DOE's Idaho National Engineering and Environmental Laboratory (INEEL) announced in late June that it completed one million miles of hybrid electric vehicle testing in fleets located in Arizona. Eighteen hybrid vehicles, including all four models currently available in the United States, were used for a variety of purposes in several fleets, and the cumulative mileages for the four vehicles are: 38 miles per gallon (mpg) for the Honda Civic, 46 mpg for the Honda Insight, 41.1 mpg for the 2002 and 2003 Toyota Prius, and 44.6 mpg for the 2004 Toyota Prius. Except for one of the six Insights, the vehicles all used a continuously variable transmission, for which the U.S. Environmental Protection Agency (EPA) lists the combined city and highway mileages as follows: 48 mpg for the Civic, 56 mpg for the Insight, and 55 mpg for the 2004 Prius. In other words, the fleet tests yielded mileages that were about 20 percent lower than the EPA mileages. See the INEEL press release and the DOE/EPA Fuel Economy Guide's comparison of new hybrid vehicles.
Why the difference? As noted by Toyota in a recent press release, the EPA tests are conducted at average speeds of no greater than 48 miles per hour, without air conditioning, and at moderate temperatures. In contrast, the fleet tests in Arizona likely included higher highway speeds, with air conditioning, and at high temperatures, which can limit the effectiveness of the hybrids' battery packs. In fact, month-by-month results from the Arizona fleet tests show better results during the cooler winter months. See the detailed test results from INEEL, and for information on mileage tests, see the Toyota press release and the Frequently Asked Questions page on the Fuel Economy Guide Web site.
It's not just hybrid vehicle owners who report lower mileages than the EPA figures: A recent petition of the EPA by environmental advocacy group Bluewater Network claimed "drivers today continue to complain that they are not achieving the fuel economy displayed on the window sticker when they purchased their vehicle." In response, the EPA requested comments in late March on a proposal to revise its fuel economy tests. See the EPA press release, the Bluewater Network petition (PDF 54 KB) and the EPA request for comments (PDF 66 KB). Download Acrobat Reader.