Among the most damaging allegations in New York Times reporter John M. Broder’s now-famous review of Tesla Motors’ Model S had to do with highway speed. On the trip from D.C. to Boston in his sleek test vehicle, Broder wrote that he had to slow down to save on battery juice:
The displayed range never reached the number of miles remaining to Milford, and as I limped along at about 45 miles per hour I saw increasingly dire dashboard warnings to recharge immediately.
In a painstaking rebuttal, Elon Musk, Tesla topper, called nonsense on the limping figure: “Broder in fact drove at speeds from 65 mph to 81 mph for a majority of the trip…” That evidence came from monitoring technology on the car, a fact-checking precaution that the company installs specifically for media types.
To that challenge, Broder, in turn, unspooled the following argument, in part:
I cannot account for the discrepancy, nor for a later stretch in Connecticut where I recall driving about 45 m.p.h., but it may be the result of the car being delivered with 19-inch wheels and all-season tires, not the specified 21-inch wheels and summer tires. That just might have affected the recorded speed, range, rate of battery depletion or any number of other parameters. Tesla’s data suggests I was doing slightly more than 50 over a stretch where the speed limit was 65. The traffic was heavy in that part of Connecticut, so cruise control was not usable, and I tried to keep the speed at 50 or below without impeding traffic.
Let’s have a deeper look at this wheel-diameter question. Anyone who made it out of 7th grade mathematics can understand how unaccounted-for differences in wheel diameter and tire circumference would well corrupt the accuracy of a car’s speedometer.
Rob Enderle, a tech industry analyst, says that changing the wheel size without a compensating adjustment of tire girth is a rare occurrence. Auto companies, he says, are well aware of how their speed-measurement systems are calibrated to a fixed tire size. “It almost never happens unless someone made a huge mistake,” says Enderle.
Let’s just say someone made a huge mistake in this instance, slapping a new wheel-tire combo on Broder’s test-drive vehicle. So now you have a pricey Model S chugging up the Eastern seaboard showing an inaccurate readout of its speed.
So what? The spat between Broder and Tesla on this front relates not to how accurate the car’s speedometer is when compared to, say, the world’s most accurate radar gun. It relates only to the speeds that the car itself was reporting.
The speed chart generated by Tesla Motors (shown at the top of this post) says that it derives from “vehicle logs.” And the readings that Broder put into his review also came from the car’s own speedometer. Even if that speedometer wasn’t taking accurate measurements vis-à-vis the universal truth, it was still presumably giving Broder and the Tesla’s monitoring systems identical readouts. Says Enderle: “He should have been seeing the same thing that the car log was reporting regardless of how fast the car was going.” If so, the whole question of tire circumference and 19- or 21-inch wheels would appear to have no bearing on this entire discussion.
When asked about this stuff, Broder cited some questions of his own:
My presumption – not knowledge – is that the data comes from recording devices (“black boxes”) in the car. But I think one of Musk’s blog posts or tweets referred to “telemetry”, which would indicate (to me) they can monitor these parameters in real time. But at no time during the test did they initiate a call to me to indicate they had concerns about the way I was driving/charging the car.
A Tesla spokeswoman declined to indulge the Erik Wemple Blog in a deep dive on speedometers.
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