BorgWarner is supplying Tesla Motors with a single-speed gearbox for the electric Roadster after an earlier two-speed unit from a rival proved not to be durable.

So far, 27 customers have taken delivery of Roadsters.

Tesla engineers developed the specifications for the new gearbox which is an integral part of an enhanced powertrain with significant performance and efficiency improvements. It delivers about 30% higher motor torque on a single gear ratio, and achieves a 10% higher EPA combined range.

The new gearbox is designed for the higher peak torque levels of the new Roadster powertrain, which has increased from 286Nm (211ft lb) to 380Nm (280ft lb). The new powertrain achieves an EPA combined range of 244 miles on a single charge, up from 221 miles previously.

Roadster production began in March, and the first vehicles were built with an interim transmission design. Customers can have their powertrain upgraded free of charge.

"Last December, when the two-speed transmission designed by a previous supplier proved not to be durable, we announced we would modify our approach," said Tesla's technology chief JB Straubel. "By using a more powerful inverter and an enhanced motor design, we were able to implement a single-speed gearbox and still achieve our original performance goals. In fact, the new setup is superior in almost every way."

Tesla Motors starts 10 cars a week and each takes four to six weeks to complete. It expects production of at least 20 a week within a few months and 40 by early 2009.

See also: Q&A with Tesla's 'Car Guy'