February 15, 2013

Tesla Test Drive Controversy Data - Speed HIGH by 10% ?

[Summary - Think about the data. The speed looks HIGH by around 10%.]

I'm going to toss this post up, with some trepidation - I don't know if it's better if it remains in the low grass of my many handfuls of readers, or if I'd rue being an ant among elephants. Anyway, one-sentence intro: I've been following the issue of the _New York Times_ reporter who test-drove a Tesla electric car, had problems with driving range, and has been attacked by the company. I'm not going to attempt to summarize it all, it's well-covered. A key point was that the Tesla car's internal tracking data conflicted with the written account of the reporter.

Company: "Cruise control was never set to 54 m.p.h. as claimed in the article, nor did he limp along at 45 m.p.h. Broder in fact drove at speeds from 65 m.p.h. to 81 m.p.h. for a majority of the trip ..."

Reporter: I do recall setting the cruise control to about 54 m.p.h., as I wrote. The log shows the car traveling about 60 m.p.h. for a nearly 100-mile stretch on the New Jersey Turnpike. 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. ...

Here's where I start to think, we have an objective way to attempt to determine truth. I'm wary of teach-the-controversy and middling-truth punditry (e.g. Republicans say the Earth is flat, Democrats say it's round, we need bipartisanship - how about a compromise middle ground from both "extremes", that it's flat but has rounded corners?). Putting aside possible bad memory about the number, this discrepancy should be a critical point.

In the company's article, under "Vehicle Logs for Media Drive by John Broder on January 23 and 24", let's look closely at the first graph, "Speed" vs "Distance" There's a long flat stretch around "200 mi", of almost exactly "60 mph". Yup, that's cruise control. The reporter says it was "54 mph". Later on, around "450 mi", there's another flatish stretch of about "52 mph", reporter says "about 45 mph". Hmm ...

60 54
52 45

Just suppose, for the sake of discussion, that the "Speed" value shown on the graph should be reduced by 10% to derive the actual value. Then we have:

60 -> 54 (vs 54)
52 -> 46.8 (vs 45)

That starts to look very close. Plus,there's a long section of supposed "70mph"-ish readings at around "100 mi" that would look much better as "63mph"-ish results, given that the speed limit was 65 (granted, this isn't the strongest argument, but I'll assume the reporter would think risking a speeding ticket was a bad idea on a test drive).

I'm cautious about whether anything is due to different tires. That's tempting, but it may be a red herring. There was a comment in the reporter's rebuttal post that "The diameter for the 19" all season tire is 27.7 inches, with 755 revolutions per mile. The diameter of the 21" summer tire is 27.8 inches with 750 revolutions per mile. The difference of 19" and 21" are the diameters of the wheels. So as you based your calculation on wheel diameters and not tire diameters you won't get to the actual difference in speedometer readings. My calculation has a difference of .13%.".

Maybe someone just fumble-figured a conversion number for translating the tracking data into a figure of speed in terms of miles per hour. That is, where hypothetically they should have entered "755" (revolutions per mile), instead they might've entered something like "855". Such things have been known to happen.

In a thoughtful world, there'd be a cry of "THAT DATA LOOKS HIGH!". I'll leave it as an exercise for the reader to determined what sort of world we live in from the relative prominence of what is being cried.

By Seth Finkelstein | posted in journo | on February 15, 2013 07:42 PM | (Infothought permalink)