DETROIT—General Motors Co. (GM) is delaying plans to add a second shift at its plant that makes Chevrolet Volt electric cars.

The automaker says it has found ways to make one shift more efficient so it can produce the same number of cars as two shifts.

GM will, however, still add 300 workers at the Detroit-Hamtramck plant, but not a second shift, by the end of 2011.

In May, the company announced it would add a second shift late this year to increase Volt production from 16,000 per year to 60,000.

A second shift will be added, but not until the last half of 2012, when the plant also starts producing the 2013 Chevrolet Malibu and the Impala.

GM says the change had nothing to do with Volt sales, which have been slower than expected., suggesting demand for the car still is strong and it will stick to its plan to build 60,000 next year, which includes the Opel Ampera, a European version of the Volt.

Through September, GM has sold only 3,895 Volts, far short of its goal of 10,000.

But the car, which can run on battery power for about 35 miles before a gasoline generator kicks in, is only on sale in 27 states and should be for sale nationwide by year-end.

GM plans to add more than 200 jobs when the second shift arrives next year. The plant, which straddles the border between Detroit and the enclave of Hamtramck, now has about 1,000 workers.

In May, GM hinted it would add another 2,500 jobs once the second and third shifts were added to the plant and that it would invest $69 million for new equipment.

To accommodate the increased Volt production, GM will add a second shift at a plant that assembles Volt battery packs.

The new jobs are part of a larger GM expansion to create or keep about 4,000 jobs by investing $2 billion in 17 factories in the U.S.