When a Seattle test of Toyota Prius hybrids with plug in conversion test showed the vehicles averaging about 57 mpg, some people wondered if plug-ins were just hype.

But the cars are showing steady improvement, according to an article on Wired.com. The fuel economy during charge-depleting mode, when the electric motor is doing most of the work, increased to an average of 69 mpg.

"That was a 10 mpg increase, and that's where you're maximizing the plug-in operation of the vehicle," said Scott Thomsen of Seattle City Light, one of the agencies testing the cars.

The improved performance can be attributed to warmer weather and more vigilant driving. The 51 mpg average was recorded through the fall and winter. During cold weather, the Prius will run its gasoline engine until the catalytic converter warms up. This prevents a plug-in hybrid from running on electric power. The fuel economy changed as temperatures climbed because the engine was cutting out sooner, Thomsen said.

Driver behavior is also important. Performance increases when drivers use a light touch on the accelerator, mind their speed and plug in at every opportunity to keep the battery charged.

Thomsen notes that increase in fuel economy is accompanied by a corresponding decrease in CO2 emissions from the tailpipe.