Cottonwood
Roadster#433, Model S#S37
I agree. I've been experimenting with cruise control on basically flat terrain for a 20+ mile stretch of highway to and from work, and have noticed that with just a moderate amount of attention I can keep the orange line much steadier than cruise control will, which jitters quite a bit. My guess is that the cruise control is so finely calibrated that it tries to keep exactly the same speed, to something like a tenth of a mile per hour, over every little bump, rise, etc. which wastes energy. Having said that, I'm not sure that the difference is worth the inconvenience of the attention and the effort of feathering the pedal. Cruise control at 55mph has been getting me about 270-280 wh/m while manually I can get about 260-270 depending on the conditions I've experienced so far.
I have to respectfully disagree.
Unless the downhill puts you into a regen situation, holding a constant speed will use the lowest energy per mile over a trip, if you use the constraint of a constant travel time to achieve the same average speed over the route. Taking longer to drive the route (lower average speed) can always use less energy.
This happens because wind resistance is proportional to the square of speed and is therefore a non-conservative force; the extra that you put in going faster is not made up for going slower.[SUP]*[/SUP]
Let's take a simple example with plausible numbers. Assume a route that is 2 miles long and at a speed of 60 mph, further assume that the energy to overcome wind resistance is 200 Wh/mi at 60 mph. If I drive the route at 60 mph, wind resistance consumes 2mi*200Wh/mi or 400 Wh. If I drive the first segment at 45 mph (3/4 of 60), that segment will take 1:20 and use (3/4)[SUP]2[/SUP] or 9/16 of 200 or 112.5 Wh. To drive the second mile in 0:40 (2:00 total), I need to drive 90 mph or 3/2 of 60 mph. That will consume (3/2)[SUP]2[/SUP]*200 or 450 Wh for a total of 562.5 Wh.
This is just one example, but in this case, I used 400 Wh for 2 miles or an average of 200 Wh/mi for aero losses driving a constant 60, to take the same amount of time, I could drive the first mile at 45 and second mile at 90, for an average speed of 60 and an average energy for aero losses of 281 Wh/mi. Try other cases, but you can't beat a constant speed for minimum energy use.
There is a counter argument that because of resistive losses in the battery, inverter, and motor, which are proportional to the square of current, it is less efficient to run at higher powers. This is true, but at a very small level. Resistive losses are in the single digit percents or lower at moderate hill climbing powers. That is tiny compared to aero losses that are well over half of the energy used at highway speeds.
Slowing down will use less energy, but the best gain is had by slowing down to a constant speed and not just slowing down on the uphills.
Enjoy your driving!
[SUP]*[/SUP]This often causes confusion. Wind resistance is proportional to the square of speed, but the power to overcome wind resistance is proportional to the cube of speed. This happens because power is force times speed and energy is force times distance.