I periodically post my technique for getting better gas mileage, so apologies if some of you have read this before.
I'm gonna start off here saying that none of the below is ANYTHING like disagreement. Your Insight insight absolutely applies. Where it gets tricky is the number of OPTIONS the Volt has, not necessarily just as driver controls but programmed in deep down inside where it's hard to see what happens aside from the outcomes. There's a LOT of blackboxing (where we don't know what happens), and the drivetrain for the Volt is both blindingly simple AND insidiously complex at the same time. And there's DIFFERENT ones for Gen 1 and Gen 2.
All hybrids (and in CS mode, the Volt is essentially a hybrid) have, broadly speaking, at least two basic recharge scenarios. For the Insight, it was referred to it as "background charging" and "forced charging" or "forced regen". Background charging replaces energy via braking, coasting and running the ICE at efficient levels slightly above what is required to move the car at the selected speed.
Forced regen occurs when the hybrid system determines that the background charging is not sufficient to keep the battery sufficiently charged. Under forced regen, the ICE is tasked with running outside its efficient range to replace that charge. Forced regen absolutely kills ICE efficiency. You might as well be pulling a trailer.
On a typical hybrid, the battery budget is so small that you just deal with forced regen when it happens or use technique to make sure it never happens or (in the case of a really old Insight with an unbalanced battery pack) you hack a grid charger and periodically recondition the battery and otherwise baby the car.
The budget is, in my personal vocabulary that I don't haul out here much the "Charge Sustain (CS) window". The endpoints in the State of Charge of the CS window are "CSlow" and "CShigh". (There's also Hlow, Hhigh, and Mhigh as well. At least for Gen 1, I haven't any reason to think there's an Mlow anywhere. Or Mhigh and Mlow are exactly the same point. It's complicated.)
The Volt has a much larger battery pack than a typical hybrid, so forced regen can almost always be avoided with a little planning and technique.
Yup. It feels like that window/budget is about half a kwh. Which is basically (a small number of) miles. For normal hybrids (like a Prius) running normally, the window gives them a couple hundred yards maybe. In special Battery modes, they MAY allow as much as a Volt does as a matter of course.
VoltenRock, it sounds like you are putting the Volt in Hold mode at highway speeds and then returning to Normal. When you first engage Hold, the ICE is cold and the Volt won't immediately run at the level needed to keep the CS Hold level of charge until the ICE warms up. Whether you loose a bar or not (the display is a rough gauge), I can almost assure you that whenever you engage Hold with a cold ICE, you will loose some charge while the ICE is warming up. Once the ICE is at temperature, the Volt will go into a forced regen mode to replace that lost charge incurred during ICE warmup.
Here's where I think we start getting a little complicated. Hold Mode, at least in Gen 1 probably both, behaves a LOT like CS. I suspect that the window is the same size and all the same operational parameters are the same. It just allows drivers to invoke CS with an arbitrary Hhigh and corresponding Hlow. The Volt also
seems to have a warmup period that lasts about 30 seconds below some minimum engine temp. During that warmup period, basically no power is generated at all. (ERDTT for later (2013+) Gen 1 and Gen 2 cars seem to use this warmup mode at all ERDTT times. Early Gen 1 are ... different. They get what seems to be exactly Hold Mode, but cycled by engine temp instead of SOC endpoints.) After the warmup, the Volt switches to normal CS/H operation. When the SOC drops to the CSlow point, the Volt goes into an "engine on, passive charge" that puts out about 6kw plus whatever gets drawn off through the gears to the wheels. If the CShigh SOC gets reached, the engine shuts off until SOC is back down to CSlow. If SOC gets below CSlow, though, the Volt switches to a much more aggressive charging cycle. (~12kw over what's needed to move the car at that speed? I've got no way to measure, so I'm going by time and what modes could be reused from OTHER things that can be timed. But failing that generation rate is what pops the Reduced Propulsion Mode. And it's EASY to imagine an old battery, while getting through the warmup, draw down to below CSLow and then be not in a mechanically-linked drive mode when the demand for 15kw PLUS moving the car at high speed or accelerating, and immediately throw RPP.)
So, when you first engage Hold, the ICE is going to be inefficient due to the normal issues related to warmup typical to all gasoline engines (cold oil, tolerances off, heating up the cat, etc.) and then the ICE will be in forced regen mode to replace the charge lost during warmup.
TLDR: If you are using Hold mode for shorter distances, the gas mileage will be horrible.
Plus warmup cycle. You're probably not going to get more than one of those on a single trip, but popping in and out of Hold on a cold day with more than a few minutes in between might.
I have two suggestions for you.
First, reset the Hold mode when the ICE gets to operating temperature. It takes a few minutes at highway speed or you can switch the display to engine temp and wait for coolant to get to about 180 degrees (final operating temperature is generally higher, but it seems that the Volt will allow the ICE to completely take over somewhere in the mid-170 degree range). To reset the Hold mode, just briefly switch back to Normal and then reengage Hold mode.
I'm not sure that fits well with my model of How Things Operate. Not that it'll do any harm, but I'm not sure it'll do any good either. But I'm willing to take "You got a 12. What do you know, Mr No Hold Mode At All?" If it DOES start the engine under all conditions (even sitting someplace stopped but in Drive), it may indicate that turning on Hold sets Hlow instead of Hhigh, and immediately kicks the car into the background/passive 6kw charging mode for a few more minutes.
Second, plan your Hold mode usage to keep the number of warmup cycles to a minimum. If you have a commute that is a little too long for Normal mode to handle on its own, perhaps you can use Normal all the way to work, and then use Hold mode for the majority of your drive home. If you go from two warm up cycles per day to one, your gas mileage will improve.
I got one more: That warm up period may be wasted gas but it's not very MUCH wasted gas. If you're on the highway, with 5 miles to your destination and 4 miles remaining, resist the temptation to pop into Hold. You may well make it anyway because that last mile on city streets will come at 30 MPH of wind resistance and it's not easy to tell precisely where you are in the CS window. especially at highway speeds where the engine is likely powering the wheels too. Half a kwh may be as much as two miles at low speed and street level, and Hold Mode will allow that much variation. It's way better in aggregate, to always presume you're going to make it, then lose sometimes, than to presume you won't, burn a pint of fuel always, and then have it turn out you would have made it sometimes.
As always, none of these techniques are required. You can just drive your Volt in Normal and everything will be just fine, but if you are selectively engaging Hold mode without paying attention to some of these factors, then it is possible (perhaps even probable) that you are getting worse gas mileage than just keeping the car in Normal.
(And that's not even getting into the "what about Mountain Mode?" question. Which as far as I can tell always presumes that anything below its shutoff SOC is always going to use the more aggressive 12kw recharge cycle. But that does circle back to why I think the more aggressive charge cycle is 12kw net input. And that's because the amount of power that the Volt puts back in from CS to MM full when changed to MM in a driveway is about 12kw. The Gen 1 gets about 4.5kwh of usable charge on the battery in about 20 minutes, and Gen 2 the Gen 2 smaller amount in correspondingly less time. And I remember seeing someone showing a 6kw gain at a stoplight in CS mode, so that's my support for that figure in passive "I'm just gonna build up this window" charging.)