Mileage - Cruise vs Adaptive Cruise

[StuartV]

I have about 8K miles on my truck now.

I use Adaptive Cruise Control all the time. Like, going down the 40MPH street that leads to my neighborhood, I'll have it on. There is always traffic around here, so I have found it very easy and convenient to just let the ACC manage my speed and keep me the right distance behind whatever vehicle is in front of me. Of course, I use it all the time on the highway and Interstate, as well. Those are also normally anywhere from moderately congested to stop-and-go. I live in the DC metro area...

My fuel mileage has averaged 14.5 MPG over the time I've had the truck. That is running 89 octane gas and with road trips to Pittsburgh and 4 times down to North Carolina. So, definitely not all city driving.

I have had a theory that the way ACC works has been hurting my mileage. It frequently actually brakes when, if I were managing the gas pedal myself, I would have just let off and let it coast down. Then, of course, the ACC has to accelerate more, once traffic picks back up, because it slowed down more.

So, yesterday, I was coming home from Wilmington, NC. I stopped early in the trip and filled up. I reset the tripmeter and fuel economy display. After the fill-up, I was on highways where the speed limits were generally 55 - 65. I drove at least an hour on those roads, using the ACC - always set at 10 MPH over the limit. After an hour (or maybe 2), my lie-o-meter was showing something like 15.3 MPG.

By then, I was on I-95 rolling north. I switched it from ACC to regular cruise control going the same speed and left it that way for the remainder of the ride home. By the time I got home, the lie-o-meter had gone up to 17.0 MPG!

It's not very scientific, at this point, but I am just THINKING that regular CC gives at least 1 MPG better fuel economy than ACC. Maybe more like 2 MPG.

 

[GRAYWOLF]

My computer estimated MPGseems to vary from calculated (miles driven ÷ gallons to fill up) MPG up to 1.5 mpg

Sent from my SM-G965U using Tapatalk

 

[Richard320]

I think your hyposthesis is correct. Unfortunately, I don't have ACC to try the experiment myself. But it does make sense. Using standard cruise control on the highway, I often pass someone only to have them accelerate and pass me a mile or two later, and then repeat again and again. Why? Because they aren't using cruise control, so they slowly decelerate and then use a bunch of gas accelerating back up to beyond the speed limit, whereas if the speed had been kept steady, they'd get much better mileage. The ACC is doing the same: slow down, accelerate, repeat.

I really like the cruise control buttons on the wheel, especially the - button. If I can see I'm closing the gap on someone, I can just click it once or twice and slowly decelerate to match their speed. ACC would likely keep me at a steady speed until I got to a certain distance and then brake. And then try to accelerate if the gap increased again.

 

[StuartV]

My computer estimated MPGseems to vary from calculated (miles driven ÷ gallons to fill up) MPG up to 1.5 mpg

Sent from my SM-G965U using Tapatalk

Mine very from hand calculated by as much as 2 MPG.

I use Fuelly to track every fill-up in my truck. I reset my tripmeter and Fuel Economy meter at every fill-up. And I put in the notes in Fuelly what the displayed FE was.

I filled up the other day after putting 473 miles on a tank. The onboard computer was showing that I had been getting 17.2 MPG. The hand (Fuelly) calculation was that 31.33 gallons meant I actually got 15.1, not 17.2.

That was the truck saying I was doing way better than I really was. Other times, it's pretty close to spot on. Every now and then, the hand calculation will come out higher than the lie-o-meter. But, so far, that has only happened when I filled up after only driving a short distance. E.g. I topped up before starting a road trip.

Any time I put 20 or more gallons in the tank, the calculation almost always comes out that the lie-o-meter is 0.5 - 1.5 MPG higher than what I really got.

 

[Captain Stack]

I agree. The adaptive cruise control is a MPG killer. But more importantly for me it is a BRAKE killer. I usually go 70-80k before I need brakes and I think the way ACC is using the brakes they won't last 30k!

I do still use ACC in heavy stop and go traffic though as it keeps me safe from rear ending someone.

 

[GerryS]

Before giving up on ACC, have you tried the three different sensitivities? Maybe using the highest sensitivity will reduce brake use?

Gerry

 

[SpeedyV]

Before giving up on ACC, have you tried the three different sensitivities? Maybe using the highest sensitivity will reduce brake use?

Gerry

The problem with the higher sensitivities (in the city) is that you set yourself up for being cut off by drivers that see you leaving space in front of you...which then results in your Ram hitting the brakes harder to maintain the gap after someone cuts in. Ask me how I know

 

[StuartV]

Yeah, even with the closest following distance, people still cut in...

 

[Cueva del Osos]

I drive mostly mountain highway miles and use ACC on the state highway (getting to the interstate) to avoid another speeding ticket from our local state troopers (got nailed a few months after moving here - I wasn't paying attention on a downhill curve). I find the ACC gives me an MPG boost on that stretch and figure it's due to the rolling terrain...my foot can't possibly keep speed as steady as the ACC. When we tow, I'm maxed at 65 mph by trailer tires (and safety concerns...any faster and trailer can get a little less stable) and I typically run ACC so I can enjoy the drive. I find I get better mileage in that situation as well. Of course, with ANY cruise control, I drop out on steep uphill grades to avoid the inevitable rocket booster effect. I don't like to feel like I'm trying to achieve escape velocity! lol

 

[Cortesio]

I think your hyposthesis is correct. Unfortunately, I don't have ACC to try the experiment myself. But it does make sense. Using standard cruise control on the highway, I often pass someone only to have them accelerate and pass me a mile or two later, and then repeat again and again. Why? Because they aren't using cruise control, so they slowly decelerate and then use a bunch of gas accelerating back up to beyond the speed limit, whereas if the speed had been kept steady, they'd get much better mileage. The ACC is doing the same: slow down, accelerate, repeat.

I really like the cruise control buttons on the wheel, especially the - button. If I can see I'm closing the gap on someone, I can just click it once or twice and slowly decelerate to match their speed. ACC would likely keep me at a steady speed until I got to a certain distance and then brake. And then try to accelerate if the gap increased again.

The ACC isn't quite that jarring. In my experience, if I come up on someone going slower than I am, the truck will gradually slow down. It detects the car in front a pretty good distance away and the system doesn't seem to be entirely "dumb" with how it manages speed and following distance. Similarly, the truck doesn't race to maintain the exact following distance. If the guy in front of me speeds up, my truck gradually speeds up. Only after a significant gap has been made does the truck lay on more acceleration to get me up to the speed I have set. Having someone merge in front of me is definitely the worst because the system detects it's following too close and tries to regain the gap by heavily braking.

We need to make sure we're comparing apples to apples between ACC and standard CC. On an open road with no traffic, both will behave essentially the same with no difference in fuel economy. In contrast, you have ACC specifically for more congested traffic and would not even use standard CC. There's no comparison between the two at that point.

A better comparison is between using ACC and driving normally without any form of CC. There may be some benefit here because we are smarter than the ACC system and can parse more complex information about the specific driving conditions we are in. We can absorb following distance by coasting where ACC would brake to maintain distance. Likewise, we can typically see further traffic conditions to know if we should accelerate to match pace or if we should maintain our speed because traffic will be slowing down again. These nuances are where we would gain some MPG benefit. That said, we don't have ACC on our vehicles for a fuel economy benefit. We have it specifically because what it lacks in sophistication for differentiating various traffic conditions, it makes up for in safety by having a far superior reaction time and never having a lapse in concentration or attention.

 

[LouCap]

Does the ACC always brake to maintain distance on the highway? On several long trips, I've looked down to check my speed, and I was surprised to see that it's lower than the limit I set. So, the ACC has slowed me down without my feeling any brakes being applied. Does the system just ease up on the gas if the speed is only a few MPH slower? Will it coast rather than brake if it's not a sudden slowdown?

 

[Timeless]

Does the ACC always brake to maintain distance on the highway? On several long trips, I've looked down to check my speed, and I was surprised to see that it's lower than the limit I set. So, the ACC has slowed me down without my feeling any brakes being applied. Does the system just ease up on the gas if the speed is only a few MPH slower? Will it coast rather than brake if it's not a sudden slowdown?

I believe it depends on the situation. If the situation is right I believe it can reduce speed with just less go pedal. However it likes to use the brake if it needs more than 1 or 2 mph difference.

 

[Cueva del Osos]

My experience is that ACC does just ease off the accelerator if you’re coming up on a vehicle, but if someone pulls into your lane ahead, the ACC reads the leading vehicle’s speed and factors that into a coast or brake decision. we live in mountainous terrain and frequently you’ll pull into left lane to get around trucks laboring on the uphill grades. If you’re driving 75 in the left and some Cruze or the like pops from right into the left in front of you and then finds they can’t accelerate on the hill, the ACC will definitely brake slam you. i find that using ACC isn’t a good match to my driving when I’m actually intent on getting somewhere. When I’m just cruising without a timetable, it’s great.

 

[StuartV]

I'm going to use ACC less, going forward. But, it was a godsend yesterday driving around the Beltway and then out I-66 from DC in rush hour traffic! Sure, I had someone cut in front of me every now and then, but really not that many times. And I didn't really care! Not having to sit there jumping back and forth between the gas and brakes constantly was so much more relaxing!