it's just not possible to create an accurate estimation of range so the car's computer is constantly recalculating based on recent performance, which will very quite a lot unless you drive on level ground, the same speed and same ambient temperature each day. Folks who have MPG estimators on their car will be familiar with how much this can all vary. With a gas car, the tank is so large, we just don't have to pay attention to range down to the last mile but if you did, you'd see the number vacillate a lot as well. My understanding also is that the computer makes it's calculations based on floating voltage, which is, at best a estimate of the amount of electricity remaining. From what I've gathered so far, measuring charge left in a battery is much different than actually getting a reading of the level of gas left in an ICE vehicle's tank, there is no way to accurately measure the true amount of electricity left in a battery on the fly. the calculator does seem to get more accurate the closer you get to the end of the charge. I believe this is true because the computer is projecting the effect of current driving habits and conditions over a shorter and shorter distance (as miles left declines).
You hear many complaints about this system, but I have to say I find it valuable in letting me know how my current and most recent driving is effecting overall efficiency, if it drops a lot, it's usually because I'm going up hill or because I'm "letting loose" a bit with the accelerator or cranking the heater. I find it a good indicator of how efficient I'm being, which is helpful when I'm going far enough between charges that I have to pay attention to range to make sure I don't run out too soon. I do not expect the range indicated at the beginning of the drive to relate closely to what I will get out of the full charge, but I find the fluctuations while I'm driving to be helpful somewhat, to motivating me to using range maximizing techniques when I need to.
Gasless: Silver 2012 SL, traded in for Lease on 1/13
Tesla S P85, Gray, pano, carbon fiber, took delivery: 2-9-13... LOVE this car!
9.8 kW PV Solar installed 9/12, http://www.westseattlenaturalenergy.com
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