Picture: David Zalubowski / AP.

Palo Alto - Consumer Reports said it plans to retest the brakes on Tesla's new Model 3 after Chief Executive Elon Musk promised a software update, but the potential hit to sales from the magazine's negative review weighed on Tesla shares.

A review by the influential magazine on Monday said the car, despite many positives, had "big flaws," including braking slower than a full-sized pickup truck. That criticism adds to headaches for Musk, already facing pressure over a series of crashes, production issues, and the company's finances.

"If Tesla can update the brakes over the air - an industry first - we’d be happy to retest our Model 3," said Jake Fisher, Consumer Reports' director of automotive testing.

Musk in a tweet late on Monday acknowledged the brake issue and said that the magazine's tests had used two early versions of the car before improvements had been made.

"Looks like this can be fixed with a firmware update," Musk tweeted, saying Tesla aimed to roll out a solution a few days. "With further refinement, we can improve braking distance beyond initial specs."

Musk said he would ask Consumer Reports to retest using a more recently built car.

The Silicon Valley company uses so-called "over-the-air updates" (OTAs) to send software fixes and improvements to its cars.

Musk said the variability in stopping distance was due to a braking system calibration algorithm and could indicate some Model 3s took longer to stop than others.

"If so, we will address this at our expense. May just be a question of firmware tuning, in which case can be solved by an OTA software update."

Production bottlenecks

The Model 3 began production in July, but the rollout has been hampered by production bottlenecks. The company now plans to build 5000 vehicles per week by the end of June.

South Africa is listed among the countries where owners can place a 'reservation' for a Model 3 sedan, although no official announcement pertaining to its local introduction has been made as yet.