, that's a really good question. I regret that I'm probably not the best person to answer it. My own driving habits (in a 2018 PHEV) are such that I can go for weeks or months without starting the ICE. When I go on a long trip that exceeds my EV battery range, my overall economy is boosted by the 26+ "free" miles from the battery, but even so, if the trip is several hundred miles, I generally come in somewhere between 52 and 56 mpg. The PHEV is expected to be a bit worse on gas than the HEV, due to the extra battery weight (maybe also the electric motor weighs more?).
: interesting theory that harder breaking might be beneficial for regen: it's something I've wondered about too. We know that at some point, harder breaking engages the conventional brake pads, turning kinetic energy into friction and heat rather than electricity, but if I see a red light a half mile away and I start coasting, the "Charge" indicator on the instrument cluster just barely moves into the Charge position, and if I brake more aggressively at the last minute, it moves much further into the Charge position (albeit for a shorter period of time). So I've wondered, if my battery is close to fully charged and I'm coasting and just getting a mild charge indication on the instrument cluster, is any energy really being moved into the battery?
Classic physics concepts about conservation of energy say "yes", in order for regenerative braking to work to decelerate the car, it has to either charge the battery or else generate heat, and I don't think it generates significant amounts of heat (but it probably generates some, which is why it isn't 100% efficient). My conclusion is that aggressive breaking doesn't recover any more charge than coasting, and it runs the risk of engaging the break pads and converting kinetic energy to heat instead of electricity, but I'll be interested to hear if anyone has a different take on this..
I tend to use the adaptive Cruise Control, but I think this works against my fuel economy because it accelerates and decelerates more aggressively than I would in the traffic conditions that I'm usually confronted with. Still, it feels safer to me, because it's a "second set of eyes" watching for the need to engage in quick breaking. I think I could get better MPG if I didn't use it at all, or used it without the "adaptive" feature engaged.
In response to your request: 2018 PHEV EX Premium, 16" rims, and typically about 54 mpg on a long trip of 1000 miles with only one full charge (26 free miles) at the outset. Some of that is mountainous, some is stop and go, and lot of it is flat highway around 70 to 75 MPH. I do have the impression that 65 MPH is about optimum for highway fuel economy.