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I don't see that I have stored any energy coming off a particular mountain road with lots of curves and brakes.
 

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With our 2016 Volt I have noticed electric miles accumulated while going down hill. We have quite a few mountains here in northwestern Oregon. When running in hold mode I noticed I can climb a mountain from sea level, via Oregon Coast Highway 101, to 600-700 feet or so, at the crest I place the drive system in electric mode, from 16 miles of range on top at the bottom it would now read 20 miles or so at the bottom. Now at the bottom I place in hold mode and hit another mountain and do the same, now at the bottom of that mountain I will have 23 miles of electric range. I can now place in normal mode, electric and make it home easily which is about 18 miles away. Even having to climb another mountain on pure electric on the way home I normally arrive home with anywhere with 1-3 miles left. Usually the 130 mile round trip, Summer conditions of course, ends up being 50 mpg on gas and 65-70 miles on electric.

How many cars today can do that?
 

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Similar experience with my 2017 Volt on our first trip towing a small aluminum trailer with 2 kayaks and 2 standup paddleboards. Round trip was 161.3 miles and average mpg was 47.6 mpg. Got home with 1 mile ev range left. So far, I really love this car.
 

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Keep two things in mind:

1. It’s the same motor/generator under the hood that both pushes the car and creates the regen, and you can’t do both at the same time.

If your foot is on the accelerator as you head down the hill, either to maintain control because the road is very curvy. or the slope is not steep enough for gravity alone to pull the car down the hill at the desired speed, you’re not getting any regen at the same time, and overall downhill regen is less. With the foot on the accelerator, the motor provides propulsion torque to the wheels via the drivetrain. With the foot "off the gas" (so to speak), the car’s momentum turns the wheels, which, via the drivetrain, can use the motor as a generator. The computer can adjust the electric circuits to adjust the rate of generation, which changes the amount of the car’s momentum being used to turn the generator. The greater the regen rate, the faster the loss of momentum (think regen in D vs L or paddle vs D).

2. The computer creates an "on the fly" ev range estimate as you drive. Downhill driving is more fuel-efficient than level or uphill driving. The increased range estimate at the bottom of the hill is, in part, because your computer thinks you are going to keep driving downhill, and so can travel further on the remaining amount of power. It not necessarily because the downhill regen added more fuel into to the battery than was used during the drive down the hill.

For example, in good weather, my ev range estimate may increase from 40 to 41 miles by the time I reach the grocery store 4 miles from home that is at a lower elevation. My usage screen tells me I’ve actually used a net 0.4 kWh of power to get there...
 

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And, isn't it also true that the computer "guess-0-meter" takes into consideration your driving patterns from previous trips and thus if you drive very conservatively your ev range may continue to increase somewhat? My EV range after a full charge has been steadily climbing and I would think some of this (in addition to warmer weather) has to do with how careful I drive.
 

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Try doing the same trip avoiding the regen paddle and only using the brakes, low, or regen when you really need to slow down. Momentum is your friend, maximizing regen is your enemy, and you might find that coasting down a hill building up momentum (assuming you don't hit dangerous speeds to lose control) might get you more range than pulling the regen paddle all the time.
 

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When I coast down the hill to my driveway in my Gen 1, I can see the tenths of a kwh used kick down on the energy use screen.
 
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