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From personal experience you would need to charge from Rancho Supercharger. You can trust the % going back and forth which shows on the navigation. As far as coming down the regen works amazing and your consumption can almost be nothing or even positive.

I drive a 75d also, so whenever I go through mountains I overcompensate and as a rule of thumb charge an extra 10-15 mins even after it states it’s ok to proceed.
 
I just did a trip from Rancho Cucamonga to Lake Arrowhead and Running Spring on our MX 75D.

Started at 100% = 236 rated miles
Got to Lake Arrowhead Village: 160 rated miles
Drove to Snowdrift Tubing at Running Spring and back to Lake Arrowhead: 128 rated miles
Parked overnight (snowing in AM); 125 rated miles
Drove back to Snowdrift Tubing snow/ice road: 86 rated miles
Drove back to Rancho Cucamonga (charging downhill): 80 rated miles

MX_Snow.jpg
 
A big bear supercharger is currently the #1 requested charger in the world.
Please vote to keep it in the #1 spot.

I added 2 more specific location requests in big bear for the next voting cycle at these locations. Maybe if more people add these spots manually this will increase the chance further of getting a charger installed here.
42170 Big Bear Blvd, Big Bear Lake, CA 92315
630 Knickerbocker Rd, Big Bear Lake, CA 92315