Although city officials in Seattle say they are happy with the more than a dozen Toyota Prius hybrids in its fleet with new plug-in technology, the vehicles are coming nowhere near the claims that they would get 100 mpg using battery packs installed in the trunk, according to The test hybrids have been equipped with the Hymotion aftermarket plug-in conversion system from A123 Systems. 

Although the plug-in Prius hybrids have used less gas and emitted less CO2 than their conventional counterparts, the vehicles are averaging just 51 mpg. Seattle has totaled more than 17,000 miles testing 14 plug-in hybrids in the past year.  

Although careful drivers have achieved 100 mpg in plug-in hybrids, the Seattle fleet test and others suggest that "real world" may be far smaller. The results may point to the many non-technological factors that come into play in wringing better fuel efficiency from plug-in hybrid cars, such as driver behavior. 

Seattle decided to try the technology in March 2008 after the U.S. Department of Energy agreed to subsidize the program. The city maintains a fleet of 300 conventional Prius hybrids and converted 14 of them to plug-ins. 

General Motors and Toyota are among the automakers promising to have plug-ins on the road as early as next year. The results of these early fleet tests suggest EV advocates and automakers will have to tell people how to drive the cars most efficiently because they may not catch on if consumers don't see the fuel efficiency they're promised. 

Idaho National Laboratory is seeing similar results with the 104 plug-in hybrid vehicles it is monitoring in 22 states. The cars have logged more than 300,000 miles in the past year and returned an average of 46 to 51 mpg. 

EV advocates say the drivers aren't being told how to maximize fuel efficiency, and they don't care because they aren't paying for the gas. To get the most from plug-in hybrids, they say, use a light touch on the accelerator, lower your speed, and plug it in at every opportunity to keep the batteries fully charged. 

Keeping the cars charged is also key. If the battery runs down, the gas engine must work harder-the battery becomes dead weight-and that cuts efficiency. Seattle officials discovered the plug-ins were being driven with dead batteries nearly one-third of the time. The cars with fully charged batteries got 50 percent better fuel economy than those with dead ones. 

Plug-in advocates say Seattle's fleet results are no reason to pull the plug on the cars. The technology works, they say, and it will deliver on fuel economy, but only if people are instructed on how to get the most out of the cars.