Life with Tesla Model S: musings on new Tesla Roadster


2020 Tesla Roadster

2020 Tesla Roadster

Enlarge Photo

Like almost everyone else, I was blown away by Tesla’s surprise introduction of a second-generation Roadster at the recent semi-truck reveal event.

Zero to sixty in 1.9 seconds, a 620-mile range, gorgeous sleek lines, and a price of $200,000. Mind-blowing indeed.

But a few of the new Roadster’s numbers didn’t add up for me.

DON'T MISS: Life with Tesla Model S: assessing my new 100D vs old 2013 electric car

First, that 620-mile range, by far the longest of any production electric car.  Some critics doubt that such a thing is even remotely possible.

But my response was just the opposite: why isn’t it 800 miles?

Weight and drag

The new Roadster has a massive 200-kWh battery, twice the capacity of my Model S 100D. Yet it has less than twice the range of my car, which is EPA rated at 335 miles.

2020 Tesla Roadster

2020 Tesla Roadster

Enlarge Photo

That means a small two-seat sports car is actually less efficient than my big five-passenger sedan. How can that be?

Assuming a battery weight of 1500-2000 pounds, I’d estimate the Roadster’s weight at 4,000 pounds. That’s about 20 percent less than my 100D.

Besides weight, the other main factor that determines efficiency is aerodynamic drag, which is the product of frontal area and drag coefficient.

READ THIS: Tesla Roadster is back: 0-60 in 1.9 seconds, 620-mile range

The Model S has a drag coefficient of 0.24, which is very low.  Based on the Roadster’s similar sleek profile and Tesla’s fanaticism about aerodynamics, I’d presume the Roadster’s Cd is in the same ballpark. 

With a similar drag coefficient, the total aero drag of the Roadster compared to the Model S would then be proportional to the frontal areas of the two cars.

My rough eyeball estimate:  The Roadster  has about 20 percent less frontal area than the Model S—and therefore, 20 percent less total drag.

2017 Tesla Model S 100D [photo: David Noland, owner]

2017 Tesla Model S 100D [photo: David Noland, owner]

Enlarge Photo

My question: how can a car that’s 20 percent lighter than a Model S, with 20 percent less drag, be less efficient?

Tires

I’m guessing that the tires play a role.  The Roadster’s tires will be presumably optimized for dry traction and structural integrity at 250-plus mph, not efficiency.

The high-performance 21-inch tires on the Model S cut its efficiency by 5-12 percent compared to the standard 19-inchers. 

 

Could the extreme track-style tires of the Roadster cut its efficiency by as much as 20-30 percent?

Perhaps some tire experts can weigh in on this topic.

Excess range

My second question: why such a huge battery?

Elon Musk has often said that  there’s no need to have a range of much more than 300 miles, and I agree with him. The Roadster could achieve that with a battery half its size.


 
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