The automaker now says the Volt can go 25 to 50 miles on battery power, compared with the 40 miles it had previously said it would travel.
The distance will depend on temperature, terrain, driving technique and the age of the lithium-ion batteries.
Spokesman Rob Peterson said GM is not backing away from the original 40-mile range, and that it made the change because the company now has more experience driving and testing the Volt than it had in the past.
Drivers with a more "miserly touch" on the accelerator pedal could get better than the 40 miles, Peterson said.
GM engineers have said that the distance electric cars can travel is affected by hilly terrain or whether the driver tries to heat or cool the passenger compartment.
GM disclosed the change in a filing with the U.S. Securities and Exchange Commission that was related to its upcoming public stock sale. The sale is tentatively scheduled for mid-November.
When the battery powers down, a small gas engine kicks in to take the car another 300 miles on a full tank. Peterson said the 300 mile gasoline range is unchanged.
GM plans to start selling the $41,000 Volt in December in California; Washington, D.C.; Austin, Texas; and the New York City metropolitan area.
They'll arrive in Michigan, New Jersey, Connecticut and the rest of Texas and New York in 2011. The Volt will be available in all 50 states 12 to 18 months after its launch.