Toyota Priuses with plug-in hybrid technology are averaging 65 mpg in combined gasoline and electric-mode testing.

"That is real-world driving," said Bill Reinert, Toyota Motor Sales U.S.A.'s national alternative-fuel vehicle manager. "I ask my guys to drive them as you drive your normal Prius."

By comparison, the redesigned 2010 Prius, a parallel hybrid with nickel-metal hydride batteries, will have an estimated 50 mpg rating.

U.S. sales of a Toyota plug-in hybrid will begin sometime after 2010.

"It would make sense that we would do it with a Prius, but we haven't announced anything as to which vehicles" will offer plug-in technology, said John Hanson, a Toyota spokesman. But any hybrid model is a possible candidate, he said.

Reinert said fuel economy for plug-in hybrids depends on the battery pack's capacity and how the car is driven. Both factors determine how many miles can be driven in electric mode.

At the Detroit auto show last month, Reinert said: "How hard do you drive it? How fast do you drive it? When the hybrids first came out, there was a huge variance in gas fuel economy, depending on your right foot and other conditions.

"That difference is just magnified, supercharged, turbocharged with a plug-in electric because how fast you go really pulls the current out of the battery. It is a big deal."

Toyota will import 150 Prius plug-in hybrids late this year for testing by universities, commercial fleets and individuals.

The vehicles will test several lithium-ion battery packs that will be teamed with a 1.8-liter four-cylinder engine.