wmcbrine, thank you for your post and subsequent update.
We've had discussions over the years about the accuracy of our Range Remaining (RR) display. In our case, we DON'T call it a GOM (Guess-O-Meter - like in a Leaf) because the i-MiEV's algorithm has now been proven to be very consistent, and which I believe to be as follows:
1. The prediction is very simply based on the moving average of the consumption of the preceding 15 miles (24km).
2. The RR display is decremented by roughly 20% when the heater is turned on (also dependent on fan speed)
3. The RR display is decremented by very roughly 10% when the aircon is turned on.
Turning on the HVAC does not affect the internally-stored motive energy consumption moving average figure.
Yes, as you noticed, changes in elevation can make a significant difference. For example, I live approximately 500' above the town I drive down to every day and invariably gain a few RR miles in the process. Thus, for me going home, getting off the Interstate driving 2-1/2 miles and climbing approximately 300', means I need to compensate for my RR reading and always leave a bit of reserve - been lucky so far, as in seven years neither my wife nor I have ever seen turtle.
Especially on highway driving, by far the greatest variable is airspeed and an 80mph run can rapidly skew things dramatically - as can bucking a serious headwind.
I've found altitude to not be too much of a problem as long as you get to go back down the hill during the trip as it seems the net loss is not much more than in level driving.
Until manufacturers include the Nav destination (which will contain the elevation changes and anticipated vehicle speeds), internal temperature, external temperature, windspeed prediction over the route, and driver's characteristics (e.g., featherfoot vs. leadfoot), the Range Remaining on any car will be just a rough estimate. In my experience, Tesla's Trip graph is very accurate because it encompasses most of those variables and thus significantly contributes to my long-distance-driving peace of mind.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV