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1) Averaging 60 mph, at 4 mi/kwh (EPA range estimate), battery depletes at a rate of ~25%/hour.

2) GM advertises up to 90 miles in 30 minutes. This equates to 45kw. So replacing that 25% of charge (60 miles) equates to 20 minutes charging.

3) If you start the trip off nearly full, and finish 1/4 full, then the first 3 hours (180 miles) are essentially 'free' in terms of extra time spent charging.

So for my 480 mile trip, that should equate to 8 hours of driving, and 1h40 in charging to gain 75kw.

In practice, things worked out a bit differently. I did in fact average 60 mph and a bit more than 4 miles/kwh, and put in exactly 76kw. I charged 4 times, always starting below 50%, never ending above 80%. But I spent 2h25 to do so (45 minutes longer than the estimate).

The reasons why it took so much longer are not complicated. To be fair it was not really a surprise to me (or likely anybody who reads these forums regularly), but I think they're worth mentioning, particularly given the number GM advertises: I never reached 45kw charge rate during any charging session and I averaged 32kw overall. Possible reasons:

1) Charging stations not capable of delivering 45kw. Not an issue in my case (all were 50kw+) but many 'fast' chargers are not that fast.

2) Battery/outside temperatures causing throttling. I don't think this played much of a role for me - temperatures were 55-75 degrees out and batteries were warm in all cases (70 degrees) - but I can't be sure.

3) Battery SoC causing tapering. I did see some of this. Although all charging sessions started below 50%, there is some modest tapering at 55%.

In short - even under good conditions, it's probably more realistic to expect 30 minutes/25% charge. I'm doing a 420 mile trip next week (over one day). I plan to spend 2h-2h30 charging (in addition to 7 hours of driving).

*Note that I had gone through Vertiformed's excellent time-to-charge graphs before the trip, so I was aware that GM's numbers are grossly optimistic, if not downright misleading.