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I am confident I can get up a big hill (6000 ft in 50 miles, going 60-65 mph) getting 2.5 mi/kwh. Going down such a hill uses no energy. So that would be 100 miles using 20 kWh, for a rate of 5. But in fact the descent would store at least 4 kWh. So the total used is 16, giving an overall average of 100/16 or 6.25 mi/kWh. This is much more than I can get on flat ground at same speeds (60-65 mph).

Data to support: I can get from W Denver to the Eisenhower Tunnel (50 miles) using 21 kwh for a 2.38 mi/kwH average. But this includes several downhills. If it was pure up, I think my 2.5 estimate would apply, and I will try this sometime from Georgetown (8700 ft) to the tunnel (11100 ft), a section with no downs at all.

I can get from the tunnel to W Denver (11100 to 5200; 50 mi) using 2 kwH. The reason I have to use the 2 is that there a long flat stretch, and several good-sized uphills about halfway down.

One possible reason for some of the gain is that we know low temps. affect battery performance. But when going downhill at 0 energy, that becomes irrelevant. But the general principle here seems to transcend this. It just appears that the loss when going uphill is not that great, and the gain when going downhill is huge.

On my own small hill: 1.8 miles. 750 feet. Down: Gain 0.7 kwH. Up: Use 1.5 kwH. So total used is 0.8. That is a rate of 4.5 mi/kWh for the 3.6 miles. I know many of you have gotten 4.5. But my 4.5 is in cold temps. My first 2500 miles here have averaged 4.0. And my typical daily rides here (15-50 miles at 30-45 mph) average about 4.1 - 4.2 unless it is very cold.

I realize that this is not fully definitive and, naturally, my next time down to Denver I will record energy used on the pure downhill 20 miles from the Ike Tunnel to Georgetown and vice versa to see if the stats hold up.