If you can figure out the frontal cross section somehow, you can
multiply that by the CD and then calculate the drag on the vehicle. You
will know quite quickly if they are full of beans or not.
With a CD of 0.4, you can't possibly get 8 miles per kWh at 63 mph
with a car of even modest dimensions. Even with zero rolling resistance
and 100% electrical and mechanical efficiency. In a nutshell, you have
already spent all your energy budget on shoddy aero drag. You can't get
more than 100% efficiency out of your drive train to pull then budget
back into line.
The aero drag typically swamps all other effects on a vehicle when
you are figuring energy efficiency.
The claim that weight savings made a large difference difference in
the overall energy efficiency is silly. Yes, the rolling resistance will
increase directly with weight, but the rolling resistance is tiny
compared to aero drag. Plus the rolling resistance (roughly)
proportional to speed and the are drag is the SQUARE of speed. Thus at
an average of 63 mph, the aero drag of a 0.4 CD vehicle completely
overwhelms the rolling resistance.
The drag coefficient of a VW Golf is about 0.35. They are a "brick"
in terms of aerodynamics. In a home EV conversion with a DC drive and
lead acid batteries, these routinely achieved 4 miles per kWh, without
regen.
The GM EV1 had a CD of 0.19. It was able to get as much as 6.6
miles per kWhr.
https://www.motortrend.com/features/mercedes-benz-eqxx-gm-ev1-feature
The CD is what governs the miles per kWh, with rolling resistance
playing only a minor roll.
The electrical and mechanical deficiency are something like 90%, so
there is not a lot to gain there. A car with a CD of 0.4 and an
efficiency of 90% gets about 4 miles per kWh. If you made the efficiency
100%, you get 4.4 miles per kWh. For doing the impossible, you don't get
much benefit. However, if you instead cut the drag coefficient down to
0.2, (which you can do) you achieve your magic 8 miles per kWh. (A Tesla
has a CD of about 0.22)
Bill D.
On 12/30/2025 3:48 PM, Peri Hartman via EV wrote:
Ok, this seems slightly fishy. But from this article ...
https://carbuzz.com/renault-filante-ev-record/
... Renault claims they achieved a 621 mile trip at an average of
about 63mph (9 hours, 52 min) with an 87 kWh battery, and about 11%
charge remaining. That would be 8 miles per kWh.
So, when you look at the picture of the vehicle, you'll say "of
course, it's streamlined." But they claim the efficiency was due to
engineering and weight and that the Cd is actually 0.4 (quite high for
any vehicle). I'm skeptical of that, but then again, if you look at
some of the shots, you'll see a lot of horizontal control rods, each
of which probably have a lot of drag. So, who knows ?
If they are right, this could give great opportunity to increase range
in many other vehicles.
Peri
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