zanzabar88 said:
Is 60mph sort of the threshold for efficiency?
What are the temperature thresholds for efficiency, does it just have to be at least 50deg or so before you notice the cold hurting kW/mile?
Sorry if this has already been discussed a million times.
60 mph isn't a threshold, it's just that any amount you slow down (down to ludicrously low speeds) will be more efficient. For example, the LEAF's max. efficiency is at a constant 12 mph, which no one is going to do voluntarily (although one LEAF owner set what's still the distance record for one, 188 miles averaging a bit over 18 mph).
As to temperature, the only thresholds are related to battery heating (on/off) and the point at which a BEV switches from a heat pump (if equipped) to a resistive heater. I don't remember if the Spark offers a heat pump, but in the Bay Area climate you definitely want one if available, because it will never get cold enough to require the car to use the much less efficient resistive heater. When the winter temp drops into the mid-teens or below, a heat pump isn't going to help you at such times, but that's not an issue here.
Other losses due to cold are linear, due to things like thicker air increasing drag, the temporary decrease in battery capacity due to temp, changes in the hardness of the rubber compounds in your tires, and wet roads. Use or non-use of the heater and defroster are the biggest non-linear factors under the control of the driver. How willing is your wife to use Heat/Defrost intermittently (requiring lots of switching on and off) or do without, and put up with less than comfortable temps by wearing extra clothes instead? Heated seats may or may not be enough; women are more susceptible to cold than men, and their willingness to put up with limitations on heater use correspondingly less.
Personally, for year-round no-anxiety commutes over at least 3, preferably 5 and possibly up to 10 years, I believe a BEV in a mild winter climate like ours needs a minimum of 50% more EPA range than the un-recharged range of the commute. Thus for 72 miles between charges, you want 72 x 1.5 = 108 miles or more of EPA range (Note that the amount of city versus highway driving in the commute can alter the requirement considerably, as some cars are more efficient at highway speeds than others which have a greater combined range in the five-cycle EPA test). The only non-Tesla that's currently for sale new which approaches that 108 mile EPA range is the Mercedes-Benz B-class, but it's real-world range seems to be well under what people are getting with nominally shorter range cars. [
Edit]: Here's a useful thread for you, reporting a SJ-Monterey 72 mile drive in a B-class: http://www.mybclasselectricdrive.com/forum/viewtopic.php?f=8&t=286
The now-discontinued RAV4 EV would meet the requirement with ease, and finding a used one may be possible, although the reliability (of their Tesla-supplied components) has been a bit spotty.
Hard-core enthusiasts may be able to get by with only 25 or 33% excess range, by using the techniques described above. For those whose winter temps drop into the teens to about zero, 100% excess range is probably a good idea, and for sub-zero temps, something over 100% excess is called for.
