Tesla Motors Inc. said Tuesday it has completed the development of its new powertrain.

The powertrain uses a single-speed transmission to achieve the original performance specifications of accelerating from 0 to 60 mph in "about four seconds."

"The work is done and completed, and we are now inserting the enhanced powertrain setup into our production process," said Darryl Siry, vice president of sales and marketing for Tesla.

In December, after a series of transmission-related delays, Tesla had said it would deliver the first batch of speedy sports cars with an interim transmission that would take about 5.7 seconds to get from 0 to 60 mph, instead of the previously promised 4ish seconds, but would replace those transmissions with a higher-torque version – free of charge – once the new transmissions were ready.

The next month, the company announced it had designed a new one-speed transmission system that would solve its problems, and earlier this month, Tesla said it would introduce the new transmission system when it enters full production by September (see Tesla Announces New Transmission and Tesla Production Slower Than Expected).

And in May, Tesla Chief Technology Officer J.B. Straubel said the new powertrain was undergoing durability testing and was on track for production in August (see Tesla Powertrain Powers Forward).

Also on Tuesday, Tesla announced it has selected BorgWarner Inc. to manufacture the gearbox and has been working closely with the company since early this year.

With the new gearbox and powertrain design, the Roadster's estimated range will increase 10 percent, enabling it to drive 244 miles on a single charge. Torque will increase 33 percent to 280 foot pounds, according to the company.

"Now that we have a final powertrain design, in a matter of months there will be hundreds of Tesla Roadsters across the country," CEO Ze'ev Drori said in a written statement. "We're heralding nothing less than a new era of the automobile."

Siry added that Tesla has now delivered Roadsters to 27 customers. The company last month said it had delivered "more than a dozen" of its sporty electric cars and had 15 equipped with everything but the battery and powertrain (see Tesla Delivers More Than a Dozen Roadsters).

More than 50 Roadsters have started on the production line, and now that the powertrain is ready, the company expects to ramp up to starting 10 new cars per week from four "starts" per week today. The company plans to reach 20 per week by December and reach its full capacity of about 40 cars per week by March.

Meanwhile, photos of the much-anticipated Chevrolet Volt leaked onto General Motors Corp.'s media site ahead of schedule Monday evening (see stories from Marketwatch and Edmunds). The company promptly removed the photos, saying the release was "not intentional" but that the images would be publicly released next month, according to Marketwatch.

Photos of a car rumored to be the production Volt also came out last month, but the car turned out to be a replica of the Volt concept for the Transformers 2 movie.