As you pointed out "At such slow charging speed aerodynamics really come into play".
This includes not only your driving speed, but also altitude and the wind speed and wind direction.
Unfortunately, there are other factors that will also have a significant effect such as:
- Changes in elevation
- Temperature
- Humidity
- Rain
- Snow
- Type and condition of the tires
- Road conditions
Trying to come up with a single "driving speed" that will maximize your overall trip speed isn't going to work. If you stick with a specific calcualted speed, then sometimes you'll be going too fast. Other times you'll be going slower than necessary.
To accurately figure out your optimum speed on any give trip, you'll want to keep track of your watt hours per mile (or kilometer) as well as your average speed. Then you can multiply those two numbers and know if you are burning through electricity faster than you'll be able to restore it and can adjust your speed up or down accordingly.
Based on nearly 10,000 miles (approximately 16,0000 kilometers) of experience driving the Model 3, I'm getting a long term average of 199 watt hours per mile (123 watt hours per km). I've seen it
as low as 166 watt hours per mile (103.1 watt hours per km) at speeds of 55 mph (88.5 km per hour) in perfect conditions with a tail-wind. I've also seen it
as high as 300 watt hours per mile (186 watt hours per km)
at the exact same speed (in a rainstorm with a headwind on a trip that had a slightly higher elevation 50 feet (15.25 meters) at the end than the beginning.
On top of all of this, you may want to also consider that (depending on the length of your trip)
there may be times when the time spent charging isn't effecting your overall trip speed at all (because you were going to be stopped for that length of time regardless of whether you were charging or not). Examples include:
- Rest breaks
- Time spent at a restaurant eating
- Time spent at a hotel along the way
- Time spent charging AT your final destination