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Fuel economy worse than a big SUV

13322 Views 139 Replies 55 Participants Last post by  lesanek
Hi,

I've never owned a new car but after our Honda Odyssey gave up the ghost with 260k miles we splurged on an 2021 Ascent with 1200 miles. It was crazy expensive but everything is these days. Anyhow, I have also been Toyota guy but elected to not get a Sequoia because of the fuel economy and AWD Siennas are not easy to find. Instead we compromised on size for something more efficient (or so we thought).

We have only used the Ascent for running around our small town and the speed limit is 60mph on the hwy. It has was typically getting about 16mpg for our combined town and highway driving. I was pretty disappointed but thought we do so many short trips it wasn't probably too unreasonable.

We took our first road trip over Christmas break from Idaho to southern Utah. We have 4 kids so I had the roof top carrier on. I calculated the mileage for the whole trip and got as bad as 9mpg on a tank and as high as 14mpg. Overall, we averaged about 12mpg. I am extremely disappointed! I called two Subaru dealers to ask them if they had seen this before and they both said that they had not nor did they every hear of a service bulletin for the issue.

Its a nice car to drive around town in but man, what a gutless pig on the open road. After about 2 hours on the interstate I almost considered turning around and loading up in the F250 diesel work truck as it gets about 18 mpg on the open highway at 80mph. I felt like the Ascent just couldn't hold speed very well and was straining all the time if there was any grade. I ended up setting the cruise at 70 even though the speed limit was 80.

My thoughts are Subaru undersized the engine for the application? It's actually not up to the task of hauling 6 passengers and luggage on a road trip. Keep in mind we are a family with little kids so it wasn't terribly heavy. I tried everything from manually shifting for a few tanks to reducing speed and drafting a semi. Overall I'd say besides my old 1993 land cruiser, it was the worst vehicle I have ever driven on the interstate for performance and fuel economy. Oh how I missed the ratty old Honda mini van. It would do 90mph if you weren't careful and pull 20mpg.

I'm either super unlucky or somebody else has to be experiencing this too. I think I'm going to trade it off even though I will be taking a big hit.
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At the moment, I'm having to use my Ascent for "everything" including the many short trips to deal with our older daughter's transportation needs because the Forester is in the body shop. This is for a a fix from a minor accident that Professor Dr. SWMBO had before she started abstaining from driving for health reasons. I can actually see the fuel economy ticking down a hair on the Ascent because of those short trips for sure and a few trailer runs recently for materials and to help the other daughter and her SO pick up a piece of furniture from along the street in town. :) This is despite having a VERY light foot. It is what it is. Cold mornings + short trips = a hungry engine.
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I accept the EPA numbers for what they are, but if I drive the same way in different cars then the corresponding mpg is relative to them all. The Ascent has a harder time hitting the numbers than other cars we've had and still have. It's also something I worry little about.

Right now, with nothing but mostly short runs around town we're in 12-14 mpg range. We might only go through 4 - 5 tanks for the winter
Yep, the Crosstrek loaners and the OB 2.5i loaners I borrow are easier to make their rated MPG (or get closer at least) - in that order from easier to more difficult.

I think the heavier the car gets, the more difficult it becomes, which makes sense, since inertia is a horrible thing on gas mileage.
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I think the heavier the car gets, the more difficult it becomes, which makes sense, since inertia is a horrible thing on gas mileage.
I think this is true to an extent.. Especially with a smaller engine. On the flip side, a high powered engine on a heavy car is kind of barely working...especially with a highway rear gear. My experience shows these high powered cars easily hitting the EPA numbers.
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I'll readily admit that I wouldn't get the 12-14 mpg we're getting on the Ascent if driving the Hellcat the same short run way. Overall though it gets excellent mpg for its power and weight (around 4500#)
Except bigger engines still use the same amount of gas to do the same effort. As a matter of fact, some smaller engines are more efficient. A place this is readily visible is in standby generators for a home or construction site.
Except bigger engines still use the same amount of gas to do the same effort. As a matter of fact, some smaller engines are more efficient. A place this is readily visible is in standby generators for a home or construction site.
I think the tachometer tells the tale. The less RPMs to do the job, the less gas. The easier time an engine has to maintain stoich because of massive torque, the less gas it uses. 2000 RPM is often thought of as somewhat of a mpg borderline. 2000 RPM in our stock Viper was 100 MPH. 2000 RPM in the Hellcat is around 90 mph in 8th gear. I doubt need much gas in high gear in either of those vehicles to have as much acceleration available as the Ascent has at WOT. So saying, I'm still operating mostly in closed loop and using less gas. Both cars are inducted as well.

I do agree that smaller engines can be more efficient, but that may be less evident in a heavy vehicle needing more power applied to obtain the same amount of work a larger engine does somewhat effortlessly
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I think the tachometer tells the tale. The less RPMs to do the job, the less gas. The easier time an engine has to maintain stoich because of massive torque, the less gas it uses. 2000 RPM is often thought of as somewhat of a mpg borderline. 2000 RPM in our stock Viper was 100 MPH. 2000 RPM in the Hellcat is around 90 mph in 8th gear. I doubt need much gas in high gear in either of those vehicles to have as much acceleration available as the Ascent has at WOT. So saying, I'm still operating mostly in closed loop and using less gas. Both cars are inducted as well.

I do agree that smaller engines can be more efficient, but that may be less evident in a heavy vehicle needing more power applied to obtain the same amount of work a larger engine does somewhat effortlessly
Fuel in, energy out. Big engines loafing can be more efficient than small engines working hard. Lots of other factors too, I had a big Impala SS, about the same weight as the Ascent, with a 350 LS1 V8, (but with better aero). I could squeeze 32+ MPG if babied on the interstate.
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I think the tachometer tells the tale. The less RPMs to do the job, the less gas.
Mostly but not totally. The computer system seeks to find the optimal combination of RPM and ratio and in some circumstances higher RPM and a lower ratio is "more efficient" than lugging. That said, sometimes I feel that the Ascent's software overshoots on this, particularly on grades at lower speeds, especially when not fully warmed up.
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Mostly but not totally. The computer system seeks to find the optimal combination of RPM and ratio and in some circumstances higher RPM and a lower ratio is "more efficient" than lugging. That said, sometimes I feel that the Ascent's software overshoots on this, particularly on grades at lower speeds, especially when not fully warmed up.
I agree with you, Jim. The CVT tranny likely allows for better matching, although most all new auto trannys are very well programmed and completely seamless in shifting. The human head has no idea what the front O2s are sending up to the PC. Extremely careful monitoring of 'instant' mpg would help. That guage is based on the ever changing short and long term fuel trim values sent up from the front O2s (there could be other parameters included here as well). Tank level is another input the PC takes into account.

My point was that all PC modification is based on trying to accomplish stoich AFR. The smaller engine in the Ascent is probably in open loop (factory spark / fuel mapping) more than a big engine when trying to do the same 'work' Open loop will always burn more gas. I can still get my EPA rating in the Hellcat at 80-85 MPH...and even a bit of playing once in a while :) I think two factors are mainly responsible for that. Massive TQ mated with a 2.62 rear gear. The car stays in closed loop much more of the time than the Ascent
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The smaller engine in the Ascent is probably in open loop (factory spark / fuel mapping) more than a big engine when trying to do the same 'work' Open loop will always burn more gas.
I've never programmed or looked at the code for a modern ECU but I believe that Open Loop mode only lasts until the cat's sensors are warm enough to send accurate O2 data. I've never heard of a fully warmed, operational engine switching back to Open Loop unless a data feed is out of spec and pushes the engine into limp home mode. Maybe the process has changed or we have different definitions for open loop.

Having said that, I live in a hot climate and even when the temps are above 80°, the engine isn't fully warm until it's run for 10 miles or so (I consider 200° oil temp to be full operating temp for the Ascent). If the Ascent stays in Open Loop for those 10 miles then it does indeed use Open Loop alot.
I think the tachometer tells the tale. The less RPMs to do the job, the less gas.
That's actually not accurate or in any way relevant. That ALSO surprised me too when I learned it and started doing the math. There's a sweet spot that's different depending on where the engine produces the most power or torque required for the load.

For this one can again look at home generators. They all run the same speed in each class. So, for instance, 3,600 RPM for many home generators. Comparing, one will find that same RPMs aren't the solution or cause.

There's so many factors involved.

Look at a generator and look at how much gas it uses at whatever power output (rating is usually 50%).

So, as a for instance, the Subaru H6 3.0R was HORRIBLY LESS efficient than the FA24F in our cars, doing the same work. And the H6 3.6 was probably WORSE.

Here it is dragging around the LIGHTER Subaru Tribeca.
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That's actually not accurate or in any way relevant. That ALSO surprised me too when I learned it and started doing the math. There's a sweet spot that's different depending on where the engine produces the most power or torque required for the load.

For this one can again look at home generators. They all run the same speed in each class. So, for instance, 3,600 RPM for many home generators. Comparing, one will find that same RPMs aren't the solution or cause.

There's so many factors involved.

Look at a generator and look at how much gas it uses at whatever power output (rating is usually 50%).

So, as a for instance, the Subaru H6 3.0R was HORRIBLY LESS efficient than the FA24F in our cars, doing the same work. And the H6 3.6 was probably WORSE.

Here it is dragging around the LIGHTER Subaru Tribeca.
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Good points. In the case of small generators, 3600 RPM is the speed that produces ~120V. As the electrical load increases, the engine will open the throttle to maintain that speed, but use more fuel to sustain 3600 RPM. Inverter type small generators don't require that constant RPM to produce 120V. That's the reason that diesel OTR trucks have up to 21 gears to keep the engine in the optimal TQ range, producing the max efficiency (6 MPG hauling 80,000 lbs is pretty impressive). If Subaru eliminated the fake shifts and used only the pure CVT mode, I think our MPGs would benefit. Jason Fenske, the "engineering 'splainer" did a Youtube vid on CVTs that was very interesting.

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Good points. In the case of small generators, 3600 RPM is the speed that produces ~120V. As the electrical load increases, the engine will open the throttle to maintain that speed, but use more fuel to sustain 3600 RPM.
Yes, and now compare the watt outputs of different sized engines on the generators. Bigger engines are not necessarily better. They're often worse. The gap shrinks under higher load, but still, the smaller engines are often still better.

In our case, the FA24F obliterates the bigger engines it replaces, not just in power, but also in fuel economy. That's either at higher load (eg: city driving's inertia overcoming stop and go driving) or under lower load (highway), and the stats show that.

If Subaru eliminated the fake shifts and used only the pure CVT mode, I think our MPGs would benefit. Jason Fenske, the "engineering 'splainer" did a Youtube vid on CVTs that was very interesting.
Absolutely. And performance would increase. In that WRX, it would be substantially faster than the 6 speed version. Those of us who've torque launched our Ascents have seen that too.
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I've never programmed or looked at the code for a modern ECU but I believe that Open Loop mode only lasts until the cat's sensors are warm enough to send accurate O2 data. I've never heard of a fully warmed, operational engine switching back to Open Loop unless a data feed is out of spec and pushes the engine into limp home mode. Maybe the process has changed or we have different definitions for open loop.

Having said that, I live in a hot climate and even when the temps are above 80°, the engine isn't fully warm until it's run for 10 miles or so (I consider 200° oil temp to be full operating temp for the Ascent). If the Ascent stays in Open Loop for those 10 miles then it does indeed use Open Loop alot.
Open loop on start up is basically the equivalent of a choke....providing more gas than air. According to several here the idle stays high longer to get the turbo up to more efficient operating temp. Because the engine doesn't need 'full choke' for longer than several seconds, its possible the higher idle is operating in closed loop, ...but once again higher RPMs are using more gas as noted by all talking about start up gas usage. On most cars, the idle will drop back in 10-15 seconds or less depending on ambient temp. The pc then switches to closed loop and takes front O2 fuel trim input... always looking for stoich. I would note our inducted Hellcat and Viper don't work that way. Both those cars come off of open loop shortly after start up. You can hear it and see it on the AFR gauges

To my knowledge, at a predetermined pedal push, or especially WOT, the car goes into open loop, throwing out the fuel trim input from the front O2s. Once the 'event' is over, it will go back to closed loop and search for stoich. Open loop provides set factory spark and fuel mapping which is not fuel efficient at all. All cars are programmed this way. I'd be surprised to know that's no longer case.

That's definitely one of the reasons my high powered cars easily obtain EPA ratings at 80 mph or higher. They're not only in closed loop more, but the rear end / tranny ratios let the operate at lower RPMs
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Open loop on start up is basically the equivalent of a choke....providing more gas than air. According to several here the idle stays high longer to get the turbo up to more efficient operating temp. Because the engine doesn't need 'full choke' for longer than several seconds, its possible the higher idle is operating in closed loop, ...but once again higher RPMs are using more gas as noted by all talking about start up gas usage. On most cars, the idle will drop back in 10-15 seconds or less depending on ambient temp. The pc then switches to closed loop and takes front O2 fuel trim input... always looking for stoich. I would note our inducted Hellcat and Viper don't work that way. Both those cars come off of open loop shortly after start up. You can hear it and see it on the AFR gauges

To my knowledge, at a predetermined pedal push, or especially WOT, the car goes into open loop, throwing out the fuel trim input from the front O2s. Once the 'event' is over, it will go back to closed loop and search for stoich. Open loop provides set factory spark and fuel mapping which is not fuel efficient at all. All cars are programmed this way. I'd be surprised to know that's no longer case.

That's definitely one of the reasons my high powered cars easily obtain EPA ratings at 80 mph or higher. They're not only in closed loop more, but the rear end / tranny ratios let the operate at lower RPMs
I think you would REALLY like running ActiveOBD on a phone and data logging.
That's actually not accurate or in any way relevant. That ALSO surprised me too when I learned it and started doing the math.

There's a sweet spot that's different depending on where the engine produces the most power or torque required for the load.
I think you may have mis interpreted my point, or my intent, but you completely agreed with me in your last sentence. The PC is looking for stoich the best way it can. And I still maintain that 2K RPM is a bit of a sweet spot for many cars assuming they can travel at stoich no matter the load at that RPM

RPMs at highway speed do matter... Especially in the Ascent. RPMs are directly proportional to how much gas is being consumed....even though the car is running more efficiently in a lower gear than staying in the higher gear to do the same work.

Our Hellcat gets better mpg than the Ascent at 80 mph. RPMs are absolutely part of the equation as is the 2.62 rear gear which the Ascent wouldn't be capable of having and having any sort of power.

There's a trade off between power and engine size. Typically the smaller the engine the lower the FDR needs to be.. Robbing mpgs. Tranny gearing too plays into the picture. In our Honda RL, 2K rpm was absolutely the break even point for MPGs and that was with a 3.55 FDR as I recall.... Only having a 6 peed tranny didn't help either on the G1. The G2 RL added a couple gears, but 2K rpm was again the cut off, allowing for a little highway speed before crossing the 2K barrier...but as you accurately state, in both cases the pc shifted down according looking for stoich. All cars do that.

A quick look shows the Ascent FDR at 4.44 and 8th gear at. 51. Clearly if there's much forward resistance, it can't hold a .51 OD gear long.. Dropping to the. 65 OD gear or lower to maintain higher speeds (always looking for stoich). If the work load becomes too much, the pc will go to open loop for max power. Our cars don't have that issue. Plenty of TQ everywhere while maintaining stoich at less rpms. In the 8 speed AT Hellcat, the pc will do exactly the same thing as the Ascent, if warranted, looking for stoich...it will downshift and rpms will be higher. Higher rpms use more gas even though maybe running more efficiently at the higher rpm.
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I think you would REALLY like running ActiveOBD on a phone and data logging.
No, I only boned up on this stuff when I installed the 2.4L blower on our 2000 Viper in 2006 and tuned it with a laptop. It's nothing I feel the need to monitor anymore. I'm happy to remember some basic OBD stuff 17 years later and 71 years old :)
Our Hellcat gets better mpg than the Ascent at 80 mph. RPMs are absolutely part of the equation as is the 2.62 rear gear which the Ascent wouldn't be capable of having and having any sort of power.
That's our nose causing the issues. We're as aerodynamic as a brick. Our 4.444 rear gear can handle moving 11,000 pounds at 80, pretty easily, to the extent aerodynamics allows for acceleration.

Power for power, we're more efficient than the larger engines that could have went into the Ascent. So, my point is, RPM's don't matter, because when you compare moving 6,000 pounds with a 6 cylinder (eg: the H6 3.6R) or with our FA24F, it doesn't matter.
  • Cruising, the FA24F will utterly trounce the H6
  • Mashing the pedal, the FA24F will handily beat the H6 (but not trounce it)

It's not a matter of whether higher RPMs use more gas - that's not even true and is a big misconception. LOAD factors in tremendously. I can cruise at 4,000 RPM in my Ascent and have half the gas consumption of someone accelerating at 2,000 RPM. In the case of a car, rate of acceleration must be used as a load factor.

Regardless, to do the same work, the FA24F handily beats bigger engines, because of its power curve and flat torque line. They're better than most or all comparable NA engines. They're ridiculously better than the H6's it replaced. Again, here's the not-as-heavy Tribeca with the H6 3.6R running one of the best tunes Subaru ever released for it.

Subaru dropped the bigger engines, because they are not as efficient at any RPM, and because they're not as efficient at any relevant work level.

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@Robert.Mauro

I'm not knocking the Subaru engine at all. I also don't argue the fact the engine in the Ascent is superior to the larger engines that came before. Nor am I disputing that smaller displacement turbo engines are the way of things today for many of the reasons you state, but the statements I've made about closed loop operation and open loop to arrive at optimal efficiency are true. I find it a bit of a pleasant anomaly that a 707 HP /650# TQ, 4500# sedan that can break the 200 mph barrier (few 4 door cars are able ) can equal or beat the Ascent on the highway at higher freeway speeds...or even lesser speeds when compared by percent (27% over at 63 MPH) achieved over the EPA rating. I fully understand the brick analogy. An exact comparison will never be made.

On the contrary, I'm happy with our Ascent. We bought two of them a year apart. I'd like to see a hybrid come along. IMO, Subaru is late to the game.

Reducing carbon footprint and emissions is pretty much responsible for the bevy of small displacement turbo engines blanketing the auto industry these days. (just like 0w oil) That said, it is all about the pairings made between engine, tranny, and FDR to achieve the intended purpose. It also includes the wizardry of software to meld them together properly. Although they provided the pulling capability, the number of Ascent owners moving 11K pounds would be negligible. I've never seen an Ascent pulling anything in our travels. I'll never tow anything so maybe I've sacrificed a little for those who do given the 4.44 gear? I'm sure I'm not alone. Do I care? Not at all.

I get your point, but what percent of the population cruises the highway at 4K RPM? Cruising at 4K versus accelerating at 2K in open loop aren't comparable in any way. I don't get the correlation? I might add again that all automatic cars match engine efficiency to the proper gear and RPM based on the workload. The Subaru is far from unique in that respect.

Can you show me a TQ curve graph for the Ascent engine? I'm curious how it compares to what we have. Both our SCs are twin screw making big torque pretty early on and staying pretty much level to redline

RPMs do matter. That's not really arguable. Let me explain a different way using your 4K RPM example. If you're running at max efficiency (stoich) at 4K RPM in your Ascent and I'm running at max efficiency at 2K RPM in mine...you will use more gas. The reason OD gears were added to auto trannys was solely to reduce RPMs and improve MPGs. Thus, starting 40 years ago or so trannys went from a 1:1 4th gear to 5,6,7,8,9,10 gears. This increased engine efficiency across lower speeds as well by splitting the shift points up better. They also added more variation in the upper OD gears for less RPMs and less fuel at freeway speeds. Less RPMs at max closed loop efficiency (stoich) means less fuel used. Is it possible for any car to downshift within the closed loop parameters and still seek stoich? Absolutely. but most times it will use more gas than an engine exactinf that same efficiency in a lower gear. The discussions on here about people trying to manually shift between 6,7,8 gears and beat the PC speak to your points, but if I can stay at stoich AFR in 8th gear at 80-85 mph and well under 2K RPM, I will beat the Ascent for MPG with our big V8. I have the real world experience to prove that via cross country trips in both. I continue to maintain that 2K RPM is a rule of thumb for where MPGs start to drop off for many cars. If you think the Ascent's RPM is higher before drop off in MPGs. I'm good with that too. Further up this thread, it was mentioned that start up RPMs staying up longer is to warm up the turbo.. It was also stated, and accepted, that mileage could be impacted negatively because of it. My guess is the PC is seeking stoich in closed loop for this additional high idling and RPMs
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Can you show me a TQ curve graph for the Ascent engine?
It's a flat line, from 2,000 rpm to 4,800 rpm. 277 lb-ft, kept flat by the ECU that artificially limits torque well below what the engine can actually produce.
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I had to hunt a little for this sheet on the old 2000 Viper. Pretty flat to 5K RPM. Still 520-30 pounds to the rear wheels at 5800 RPM when I shut it down. I really had no intention of jumping on the dyno, but was egged into it at a car event. I never knew the power it had...nor cared. It ran plenty strong enough for us. Still does at 126K miles. I tuned this with a laptop in 2006-07..no dyno, but for only 5 pounds of boost, its a strong one. At the crank, roughly 700HP/750 TQ from a stock 450HP/490TQ. This car has more grunt than the Hellcat. The engine technology is this car is going on 30 years old. Completely stock, it would get 24-25 MPG at almost 80 mph. We were pushing about 4000 pounds down the road on the many trips we took with it. 80 mph was 1600 RPM. ....Going to a 3.45 gear from a stock 3.07 dropped 2 mpgs. The blower dropped it 1-2 more although I have gotten 22 with it at freeway speeds and a little tailwind :) It looks like the Hellcat TQ hits a tad later, but is flat throughout too

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