The automaker says the rechargeable electric car will get an estimated 37 mpg when running on its gasoline-powered generator.
The mileage figures will be on the Volt's window sticker. It's an estimate from the U.S. Environmental Protection Agency.
Earlier in the week Nissan said its Leaf all-electric car would get an estimated 99 mpg in combined city and highway driving.
The Volt can go 25-50 miles on electricity, then a small electric motor kicks in to generate power for the car.
Both the Leaf and Volt go on sale within weeks.