2013 Tesla Model S owned by David Noland, Catskill Mountains, NY, Oct 2015
Our recent article on the reliability implications of a Plug-In America survey of early Tesla Model S owners was met with skepticism from the company's supporters, and support from its skeptics.
But in a counter-intuitive Freakonomics-style twist, it's the firm's fans who should hope the results were accurate--and its foes who should hope they were overblown.
Lies, damned lies, statistics
The earlier article outlined a Weibull analysis on Tesla Model S owner data collected by pioneering plug-in electric vehicle advocates Plug-In America.
Based on responses from 327 owners of vehicles built in 2012 and 2013, the math suggested that two-thirds would have their motor swapped by the 60,000-mile mark.
Commenter @tech01expert argued persuasively that these should not be called failures, as the company replaced components due to mild noise (which reader @Richard characterized as trivial), whereas to most people the phrase 'drivetrain failure' connotes a vehicle that can no longer run.
Several commenters also raised the point that the statistics punish Tesla for proactively replacing components before failures could occur – something the company has been known to do, to the delight of many owners.
And many, many others contended that sampling bias invalidated the whole exercise, suggesting that the pool of survey respondents didn't accurately reflect the broader Tesla ownership.
Weibull Analysis of earliest 2012-2013 Tesla Model S drivetrains [plotted by Matthew Klippenstein]
Firstly, only people who knew about the Plug-In America survey could have filled it in, and secondly, owners whose drivetrain had been swapped might be more likely to fill in the survey than those who hadn't.
These are valid concerns, and it's appropriate for electric vehicle advocates to push back against what they perceive to be FUD (Fear, Uncertainty, and Doubt).
Reading the (tea) Leafs
Plug-In America also has a survey for Nissan Leaf owners, which could shed light on how representative the Tesla Model S data might be.
If their Nissan Leaf owners survey overreported defects, we would expect it to have overreported defects in Teslas as well. (The two surveys have the same structural sampling biases, so we would generally expect both of them to overreport, accurately report, or underreport problems.)
Conversely, if few Nissan Leaf owners reported problems in their Plug-In America survey that would imply that the survey respondents were fairly representative of the overall Leaf-buying population.
By extension, that would suggest that the Tesla statistics were probably reasonably representative as well. (Consumer Reports rates the Leaf as having average reliability and a 76-percent customer satisfaction rating.)
The only reliability-related question Plug-In America asks Leaf owners is whether their batteries have been changed. At time of writing, 1 percent (5 of 517) had. Early Leaf batteries were known to have difficulties in warmer climates, a pattern reflected in the survey, as four of the five battery replacements came from vehicles in Arizona, California, or Texas (with two).
2012 Nissan Leaf
The low number of failures in the survey, which also map to the battery's known difficulties in hotter environments, suggest that Plug-In America's Nissan Leaf sample isn't grossly overestimating quality issues. Plenty of owners were also willing to fill in the survey, despite not having anything to report about their batteries.
These factors qualitatively strengthen the case that the early Tesla Model S survey findings are at least somewhat reflective of vehicles made in 2012 and 2013. That survey may overestimate failures, but even so, the error would more plausibly be two-fold than 10-fold.
And assuming the relevant drivetrain issues have been resolved--which seems a safe assumption, given the company's recent statements--Tesla's supporters would do well to hope Plug-In America's results weren't exaggerated at all.
The Freakonomics of reliability
It's been widely reported that waiting times at Tesla service centers have increased in recent months, due in part to each service center tending to more vehicles in its vicinity.
The two ways for Tesla to bring waiting times down would be to accelerate its build-out of service centers, or to reduce the number of service-center visits per vehicle per year through improved quality.
Needless to say, building out more service centers would be the far more expensive option. Even if Elon Musk could raise a billion dollars at a dinner party (and perhaps he can), successful companies prefer to spend their money shrewdly.
For now, suppose Tesla had reliability problems with its early drivetrains and that--for sake of illustration only--half of all service-center visits stem from swaps of early Model S motors.
Tesla Model S electric motor and drive unit [photo posted by user Tam to Tesla Motors forum]
At some point, all those early drivetrains will have been replaced, and service centers will be half as busy as before (on a per-vehicle basis).
That would mean the end of waiting times for customers, and would vindicate Tesla's current rate of service-center expansion. The wait times owners now experience would be a "pig in the python" aberration that will resolve itself in due course.
The worse the early-drivetrain problem is, the more dramatic the upside for Tesla and its customers.
If motor swaps were at the heart of 80 percent of visits to service centers, then after the motor-swapping campaign these would only be 20 percent as busy as before, and could support five times as many vehicles.