If we count Canada, then Real Soon Now, Soldotna Alaska to Prince George BC - 1855 miles.
Yeah- As an Alaskan, that gap really affects me. Those two stations are at the ends of the only routes connecting Alaska with the rest of the SC network. I drove my new Model S from delivery near Seattle, WA to my home, about 230 miles south of Anchorage, AK, in nine days in October 2021 via Route 37 and plan another round trip this summer/fall.
So where to locate how many new superchargers to adequately close the gap?
Hey Tesla: My priority sites to fill this, the largest gap in the N American network, would be:
1. Tok, AK
2. Nugget City/Jct 37 Services, west of Watson Lake, YT
3a. Kitwanga, BC
3b. Ft Nelson, BC
3c. Ft St John, BC
4. Beaver Creek, YT
5. Chetwynd, BC
These sites are over two different route options, thus the 3a-3c numbering. Both would shorten a nine day trip to a three day trip. The first option requires only three supercharger stations, the second requires four. They both require overnight level-2 charging between some stations, but you need to stay overnight somewhere anyway. If your car has less than about 270 miles of reliable range, there are mostly good level-2 charging locations to get you across the longer gaps.
Priorities 1-3a use Route 37 (the Cassiar Highway), which has a few overnight charging possibilities between 2. and 3a., but none of those possibilities are optimal. I used one, but it required level 1 charging for two days at a resort operating on a wobbly diesel-powered generator. There is a level 2 possibility at that resort, but it remains unexplored. HIgh-voltage transmission lines are tantalizingly close. Other locations are either not open year-round or are even sketchier. There isn't a whole lot along route 37. A good level 2 option for an overnight stay near Iskut, BC would solve this overnight issue along the Cassiar. Iskut got connected to the grid with BC Hydro in 2014. There is a campground with cabins, and a lodge there, both now have 30a-110v power outlets.
Priority 1 & 2 with 3b & 3c use Route 97 (The Alaska Highway), requiring a fourth SC location, whereas the Cassiar route only requires three (but has the overnight issue). The Alaska Highway is about 100 miles longer than the Cassiar, but more populated. There is reliable level 2 overnight charging at Muncho Lake, between Priority 2 and 3b. It might be easier to build a 4th SC station along the Alaska Highway than to get reliable level 2 charging at an overnight spot in Iskut?
For cars with shorter ranges, these still make a 3-day trip possible because there is solid level 2 charging to fill the longest gaps with 2 hours or so of charging, especially between Prince George, BC and Kitwanga, BC (302 miles), and Tok and Anchorage, AK (318 miles).
Number 4. Beaver Creek, YT, closes a 290 mile gap between Tok and overnight at Haines Junction. There is sketchy level 2 charging at Beaver Creek now. A better level 2 option would be adequate there, but even a 50 kWh "supercharging" station would be nicer. At Haines Jct, there is DCFC. However, level-2 charging there in the off-season there isn't great. It would be nice to have a solid level 2 station at a motel in Haines Jct (or see below). Currently, you'd need TT-30 splitter and extension cord to charge at two rv spots next to a gas station and walk to a motel. Regardless of the route, you need to pass through Haines Jct.
Priority number 5 splits the 272 mile gap between Prince George, BC and Ft St John to 188 and 84 mile segments. There may be already decent level 2 chargers along this route, so #5 might be even lower on the priority list. The next longest gap would be 258 miles between priority 2 and Whitehorse, where you'd eat dinner and charge at level 2 for a couple of hours before making it 96 more miles to Haines Jct, heading to Alaska. Going the other direction, you'd eat lunch in Whitehorse while charging at level 2 before making it 258 miles to priority 2. In between, Marsh Lake has a DCFC station 34 miles east of Whitehorse, a level-2 charger there might help. Otherwise it's Teslin, which is halfway, but is pretty small.
All this said: There are already several DC fast-chargers along the route. Perhaps the cheaper interim alternative would be prioritize providing CCS adapters for those of us stuck on one side of these large gaps!!