This is a debatable topic. There are a lot of influencing factors (other traffic, temperature, wind, tire inflation, load, etc.) that no two times of driving the same route with or without CC are likely to have the same energy usage and thus achieve greater or lesser mileage.
Some say they get better mileage going by foot rather than CC. This might be due to anticipation of slopes in the road and human action to press the go pedal more or less as needed. A CC will only be reactive to the moment. For example, the CC won't let off the go when cresting a hill until it senses the affect of going downhill. A person might let off slightly sooner.
Adaptive CC may slow down sooner than a person (or annoying slow down unnecessarily when the car in front moves to a turn lane and slows down). This can cause the expenditure of energy to get back up to speed. This energy will be more than the energy it would have taken to just maintain speed.
Regeneration opportunities will influence the outcome too.
Aside: my dad once suggested that since better fuel mileage is attained with a tailwind, I should always drive to him in reverse when the wind is from his location to mine.
I agree with Ayepace here. It's a debatable and debated topic. The upshot is that compared to all sorts of other parameters that matter much more, the effect of CC is negligible.
If you did control for all other parameters you might see a few of miles difference in range ... IMO no more than 5, except perhaps in highly unusual circumstances like a constant up and down course or cars in front of you dramatically changing speed all the time.
I think the only debate is whether it is worth the hassle, but there should be no question that at a given average speed and in similar conditions, proper right foot technique is more efficient than CC. As someone with many years and miles in a Gen 1 Honda Insight with an aftermarket CC installed, I've gamed this out extensively. The main issue is being able to allow speed to bleed off going uphill and recapturing speed going downhill with minimal throttle variation as opposed to CC keeping a constant speed and occasionally "punching" the throttle.
This has been confirmed by many members fo the Gen 1 Insight community who (as one would expect) are somewhat anal about milage.
For the most part, Eco mode is simply allowing a little more variation from the set speed and a less aggressive throttle input to maintain speed. I played with this on the Rostra CC that I installed in the Insight (there were dip switches to control the stickiness of the set point and the aggressiveness of the throttle input). IIRC, the max variation in the set point was +/- 4 MPH, but the system was never going to anticipate hills and I could consistently get several MPG better with my foot.
The OG Insight got such good mileage that it was easy to see a 5 or 10% increase in efficiency (5% of 60 MPG is more noticeable than 5% of 30 MPG). But even with my wife's thirsty Velar, I can get 26 MPG on the highway using my foot as compared to 22 MPH in Eco with CC at the locally prevailing highway speeds of 80ish MPH..
And this does translate to EVs based on several years in a Chevy Volt. I have less testing in the I Pace, but so far I get noticeably better range with my foot as compared to CC in Eco and I would estimate it to be closer to 20 miles of range on a full charge at highway speeds and not 5.
As always, YMMV and 20 miles of range may not be worth the effort unless it cuts out an in-trip charging session. It is tiring to really hypermile like this even though I use CC sporadically to give my leg a rest whenever I'm in light traffic with mostly flat terrain. But it's nice to know that you can squeeze out some extra range if you need to.
Not that it helps with our cars, but Porsches can opt for the Innodrive upgrade so that their adaptive cruise system understands traffic, speed limits, curves, and slopes over the next 2 miles or so ahead and will react to improve both the experience and efficiency of the drive. Of course, as with most things Porsche you have to pay a significant cost to get the ACC capability, then pay even more for the Innodrive part. A nice thing starting this year is that the Innodrive upgrade can be added as a SOTA update IF you already have the ACC part, since all of the hardware is already there and only additional software and data is needed.
I think the I-Pace will get worse miles/Kw with the cruise control on while in traffic. If a gasser is in front of you speeding up and slowing down (as in always) the I-Pace will speed up and slow down maintaining it's set distance. Regen is nice, but anytime electricity is flowing through wires while accelerating or regenning you are loosing some energy to heat. Think power plant. Power plant makes 970KWe/hr going out the wires in the generator. Only half of that ever reaches the customers meter, to collect revenue. The rest is lost in wiring and transformer heat. If you want to be efficient with cruise control, you should not be following another car close enough so that your car is engaging regen to maintain it's distance.