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In general cruise control is going to be detrimental to efficiency because the system makes too frequent and too severe throttle adjustments in an effort to maintain speed as close as possible, this is especially true when going up even slight hills where the car will apply quite a bit of throttle to maintain the set speed when usually you would try to maintain a set throttle position and allow yourself to slow a bit or maybe only apply a little additional throttle to keep your speed from dropping too much on longer hills.

In every driver's ed course I've taken, the instructor has stressed the important of "driving with traffic (flow)." When in moderate traffic a reliable way to stay with traffic flow when others are using cruise control, is to also use cruise control and adjust to whatever the others are doing.
Got to agree with Hugh here. On a freeway with lots of traffic, your obligation to other drivers is stronger than your obligation to fuel economy. If you drop 5 mph going up a hill, typically the drivers behind you will be fuming. If, on the other hand, all the traffic slows on a hill (e.g., because someone up ahead drives like @raitchison), you just have to accept that, rather than weaving through the lanes trying to go the speed you want.

I stopped using it years ago in my ICE vehicles because I can drive much more efficiently modulating the throttle myself.
Okay, let's do some science, let's imagine an ICE car, a Golf TDI weighing 3250 lbs. Slowing the car from 70 mph to 65 mph will give you 721.8 - 622.4 = 99.4 kJ. This is enough energy to raise the car a theoretical 22.56 feet. That's assuming we can perfectly transfer energy from speed to height.

Probably most climbs you do on the freeway will be more than 20 feet. Thus there is no way there is enough energy in your forward speed to really make a meaningful contribution to going up the hill.

It is true that slowing down from 70 to 65 will reduce air resistance, but this is true independent of whether you are going up a hill (for maximum efficiency just plod along at 65 the whole time). Moreover, for air resistance, driving at a constant 70 will be much more efficient than driving on the flat at 70, up hills at 65 and down them at 75.

Now of course it's certainly possible for a car company to design their cruise control to operate with economy in mind, but based on my limited experience testing it in the Bolt and other reports I've heard it does not seem that GM did so for the Bolt.
The Bolt's cruise control works exactly as it should (for a non-adaptive cruise control). It maintains speed, as it is supposed to.

You make efficiency claims, but I think they are entirely specious. The Bolt's motor is efficient across a wide range of speeds. Driving on the flat at 70 mpjh is going to use about 22 kW. Maintaining speed heading up a moderate hill might take that up to 40 kW, and heading down the other side it'll drop power to only 4 kW to maintain speed. 22 kW and 40 kW are both going to have the motor operating at about peak efficiency. Maybe it's slightly less efficient when it's only sending 4 kW to the motor heading downhill, but we're talking about insignificant amounts of energy, especially given everything else in play.

I'd require a very detailed analysis and evidence to be convinced that the “underspeed uphill, overspeed downhill” approach is more efficient than driving at a constant speed.

And even if it were more efficient (I don't think it is) and the energy savings were actually significant rather than infinitesimal (I don't think they would be), there is still Hugh's point that slowing as you go up hills impedes the flow of traffic and isn't good driving technique.
 

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I'm interested in running the air vent without A/C or heating. I turn on the air vent, which turns on the on button with the ac/heating buttons. The temp is reading 65 degF. The outside temp is about 50 degF. The mileage goes down about 30. I hit the on button to off, the vent stops and the mileage goes back up 30.
Looks like I have to adjust the temp to the outside temp to keep the mileage from going down. Does this make sense?
 

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I'm interested in running the air vent without A/C or heating.
Turns out it's different depending on your model year. I have a 2017 and run outside air all the time without any range loss. First thing on the quest to achieve this feat, is to make sure you have the auto defog setting off in settings. Humid air will invoke the HVAC to turn on the AC and/or the heater. Next, turn off your heat/AC. Turn off your recirculation. Select an air vent path for the incoming air (MY 2020 requires two to be enabled). And for full control, select your fan speed. Hope I didn't forget anything. Lot's of buttons to push, kind of how I play an Xbox. My kids called me a button masher.
 

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The good news is, at 110F the air density will be low so less air resistance. Air conditioning will run anyway to cool the battery so it might as well cool the payload too.
But then you need to figure in the resistance from all the dust in the atmosphere in Southern Arizona :)
 
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