Tesla can use fewer cells to get the same rang and not "fill" the structural pack. Remember, long-term, Tesla is battery constrained (this year they are microprocessor constrained, but only this year, per Elon).
So Tesla can make fewer, longer-range cars, or they can use less cells, get the same range and make more cars.
The 4680 is much cheaper for them to produce than traditional "wet" electrode cells.
If you want the nitty gritty, read and study the Battery Day slide deck from 2019. There so many nuggets in that thing, it's not even funny.
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tesla-share.thron.com
TL;DR version:
- MUCH lower production costs
- MUCH smaller production footprint per GWh
- Improved chemistry AND structural pack gives better thermal characteristics (this in theory should provide longer life, less battery degradation over time) - i.e. this battery should last a VERY long time compared to the 2170 batteries, which are already industry-leading.
- Potentially much longer-range car variants (if full space savings by structural battery pack is utilized for additional cells - BIG IF per my above).
Given Tesla has a backlog of orders, I would expect them to NOT produce an "extra long range" variant anytime soon, and instead focus on a 4680-based car with similar range to the current 2170-based variant.