MPG: DRCC vs Self Driving

For a highway-speed freeway trip (ie, the proverbial easy "Sunday Drive"), have you found the EV Range and/or HV MPG to be better when cruise control is on, or driving it yourself without cruise control?

Not talking about comfort/ease vs wanting to be in control -- just asking purely about MPG.

In all my years driving, I generally never use cruise control, so just curious.

But this weekend, had a chance to try out the DRCC on a longer, fast (65mph) freeway drive.

First of all, a quirk with the Prime in HV Mode:
On the Prime, I noticed when in HV, if you leave your foot on the pedal constantly (even super lightly), it will stay in "gas" mode longer, and only switch to temporary-EV spurts after a long while. However if you take your foot off the pedal for just a second, then put it back on the pedal, it'll usually switch to a temporary-EV spurt for the standard 10-20 seconds (before going back to gas). So when in HV Mode, if you lift your foot off the gas for a sec and then put it back, oftentimes a temporary EV spurt will come on for 15 secs or so. If you just kept your foot down constantly (even really light) it waits quite a lot longer before doing those short EV spurts.

I mention the above, because DRCC seemed to operate a lot like just keeping your foot lightly on the pedal (ie, the EV spurts don't come as often). When I turned on the DRCC, it took almost 4 minutes (of 65mph 405 driving) for the first temporary EV-spurt to occur. If I had driven it myself and taken the foot off the pedal for a second, then put it right back on, a temporary EV-spurt would've occurred right away. The EV spurts certainly improve your MPG -- but typically the car will then try to "recharge" the amount used up in the spurts (even if your traction battery is close to full). So not sure how much of a benefit it is.

However, it seemed to me (though I only tried the DRCC once) that self-driving did get better MPG than DRCC -- but that may be because of false factors, like when self-driving, as much as I tried to keep the speed steady, if I got to a slight incline, I noticed the speedometer dropping to 61 or so (which the DRCC did a better job of keeping steady), and I'd have to then react and bring it back up, so maybe the better MPG was because of factors like this. Still (for whatever reason) I seemed to get better MPG on my own than with the DRCC, though again if it was because of false factors like me not always keeping up the same exact speed, I'm not sure.

So that's why I'm asking others what they have found. Is it about the same? Better self-driving? Or better with the Cruise Control?

Also, a question: I assume this was just a fluke, but wanted to ask just to make sure. When I "set" my speed for the cruise control, the speedometer showed I was going 66. But the DRCC was then set (and kept it) at 65 (the speed limit of the area) instead of 66. I assume it was just a fluke (maybe it dropped to 65 for a split second that I didn't notice)... or does the DRCC not let you "set" to anything over the speed limit?

 

Glad to hear the 1mph difference isn't just my car (not a big deal, but was curious about it). As for the "manual" driving (giving more temp EV spurts than cruise control does), I seem to get better MPG that way, but it could very well be because I lose a bit of speed by doing so. I do find it interesting that if one wants, you can force a lot of temp EV driving for better MPG (after a while, the car will take more gas to bring the battery back up to where it was) or you can just leave your foot on the gas (or use cruise) with mostly gas instead of temp EV/recharge cycles...

By the way, as for hills, I found that steep hills are better in EV (going up) as long as you have enough EV range left to still be able to be in EV for going DOWN the hill later (to partially recharge it back up with "B" mode). There's a drive I take a half dozen times a year where there's a really steep hill at the end (and at the beginning when returning). I found that if I'm in HV mode, it uses a lot of gas to go up, and going down it requires constant braking so much of the benefit of going down is lost. But if I'm in EV mode, it uses more battery going up of course, but if I can still be in EV mode for the return, that trip down really helps recharge the battery back up in "B" mode (and saves from having to use the brakes as much as a side benefit).

 

I guess you're right in a way, except that if you've run out of EV range and are in HV, and go down a steep hill for a bit, if I remember right (I only did it this way once), it has to recharge to a certain amount before it will let you access it as EV again (not just for a little bit), whereas if you still have some EV range left and are in EV, then you can use every little bit of charge you get by going downhill if you're still in EV.

As far as ICE vs EV on a hill, though this is by no means scientific, I noticed that if I can do the entire trip (up and down a long steep hill) all in EV, the by the time my range runs out later on, it's only lessened by a bit (ie, the "coming down" put back not all, but much of the extra energy needed by "going up"). So if I usually get 33.0 miles on flat stop-and-start driving (just as an example) maybe I'll get something like 32.5 (or thereabouts -- this is just an example) including going up that steep hill, then going down it, as long as it's all in EV and "B" mode on the way down.

But if I did the same trip in HV mode, it would eat up a lot more gas going up the steep hill, and I don't think it would "return" as well on the way down, especially if the EV range is gone and the car is in HV mode. I could be wrong, but that's the feeling I get. Could just be my imagination