The Volt, which cost nearly $40,000 before a $7,500 federal tax credit, could take up to 27 years to pay off versus a Chevrolet Cruze, assuming it was regularly driven farther than its battery-only range allows. The payback time could drop to about eight years if gas cost $5 a gallon and the driver remained exclusively on battery power.

The Lundberg Survey, which tracks fuel prices, said in March that gas prices would need to reach $12.50 a gallon for the Volt to make sense purely on financial terms. It said the Leaf would be competitive with gas at $8.53 a gallon.