Datapoint:
I've been going up and down my local ~2500ft mountains constantly. This past weekend I went up/down back-to-back 4x for ~10k ft total.
Avg efficiency: 235 Wh/mi over ~67 miles
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Before I got my Y I thought blasting up mountains would result in horrible overall efficiency. Now it's probably the most efficient driving I can do. It's the most fun by far and I generally take the long way back home.