As many as two-thirds of the early Tesla Model S cars required drivetrain replacements within their first 60,000 miles off the lot, according to data from Green Car Reports.

The Model S, the flagship of Tesla Motor's electric vehicles, both impressed Consumer Reports and took the watchdog group aback. It did so well in performance that Consumer Reports had to expand its rating scale, but the group also ranked the EV as below average in reliability.

Consumer Reports' "worse-than-average" overall rating got more weight behind it after Green Car Reports looked at the available data on Model S cars of the 2012 and 2013 model years.

The researchers found that as many as two-thirds of the EVs needed drivetrain replacements within their first 60,000 miles, which is a rate that's about 10 times higher than the data on the 2014 and 2015 model years. It might look a big step forward, but the researchers need more information to confirm the drivetrain improvements.

According to Green Car Reports, if there are enough Tesla Model S owners sharing their driving data, it would be able to make improvements on the statistical analysis and get numerical backup to clearly assess the car's reliability.

But it isn't a case in which too few people shared. It's just that many of the people who share data on recent Model S cars still haven't even topped 20,000 miles.

It shouldn't come as much of a surprise that a new auto manufacturer stumbles early. What Tesla Motors has going for it is all the brand loyalty it has been banking with responsive customer support.

"Close communication with our customers enables Tesla to receive input, proactively address issues and quickly fix problems," a Tesla spokesperson said in response to the report. "Over-the-air software updates allow Tesla to diagnose and fix most bugs without the need to come in for service. In instances when hardware needs to be fixed, we strive to make it painless."