In reality the difference will probably depend on driving patterns and apparently pack size. Since the S has a lower coefficient of drag, it will conceivably perform closer to Roadster numbers at highway speeds like 70 mph (115 km/hr). When you're driving in stop-and-go traffic, the light weight of the Roadster will help it perform much better than the S.
So I gather the power draw expressed in wh/mi depends on the speed of the vehicle as well as the characteristics of the battery pack.
Here's the Roadster data that Tesla published, I was thinking we would need something similar for the Model S to make a reasonable comparison.
In addition to the better aeodynamics, the larger battery on the Model S also has a slightly more advanced chemistry than the Roadster that might mitigate the added weight. Regardless, I agree that it is likely the power draw of the Model S will be higher than the Roadster.
It seems even considering the larger cross-section the Model S is still measureably more aerodynamic than the Roadster.
Battery chemistry isn't going to help energy consumption. Well, maybe a little bit indirectly by not needing as heavy a set of batteries, but that's going to be fairly small relative to total vehicle weight.
I suppose we're steering off topic since presummably the study wasn't considering high speeds, but yes my thinking is that at high speeds the power drain would of course increase exponentially in both cases, but the Model S's better aerodynamics would start to narrow the power drain in comparison with the Roadster whose power drain would increase even faster than the Model S.
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