Subaru Outback Forums banner

81 - 100 of 103 Posts

·
Registered
2005 Subaru Outback XT MT / 2012 Subaru Outback 2.5i Premium
Joined
·
53 Posts
Demonstrably not true. Hyperbole, maybe?

In top "gear" ratio, the H4 Outback runs 32.2-32.5 mph per 1000 rpm (depending on OE wheels/tires), while the H6 runs 37.0 mph per 1000 rpm ... about a 12% difference. (Source: 2015 Legacy/Outback FSM, OEM tire "revs per mile" data) Thus, cruising at 80 mph, an H4 Outback would be turning ~2470 rpm, while an H6 would be turning ~2160 rpm ... hardly "half the RPM."
Next time I drive it 80 mph, I'll get you the numbers for a hill and a flat.
 

·
Registered
2005 Subaru Outback XT MT / 2012 Subaru Outback 2.5i Premium
Joined
·
53 Posts
I'm on the highway at an 80 mph average where the H6 has less friction losses [vs. the H4] ...
How so?
As displacement increases, surface area decreases
Same thing for heat losses as heat transfer occurs through surface contact
 

·
Registered
2016 Outback Premium 2.5 CVT w/EyeSight+SRVD
Joined
·
6,927 Posts
As displacement increases, surface area decreases
Same thing for heat losses as heat transfer occurs through surface contact
You disregard the fact that while the cylinder dimensions of the H4 and H6 are very similar, the H6 has six of them. That means ~50% more friction loss (at the same rpm), ~50% more heat transfer loss (at the same power level), and ~50% higher pumping losses.
 

·
Registered
2014 3.6R Limited
Joined
·
969 Posts
Pretty sure my 79 Mazda RX-7 got better fuel mileage at 70 than 55. Rotary engines like to spin!

Time for my own anecdote. Of all the cars I've owned, one of them did get slightly better fuel mileage at 70 mph than at 55: my 1969 Porsche 911S. My only explanation for the phenomenon was the unique 911S engine: 2.0 liters, 195 DIN horsepower, 7500 rpm redline, fixed cam lift and timing, mechanical ignition timing, and Bosch mechanical port fuel injection. From curves published in the Porsche FSM, it looked like engine efficiency peaked at about 4000 to 4500 rpm; peak torque ... and peak excitement! ... was in the range of 4500 to 5500 rpm. In 5th gear, the 911S ran ~20 mph per 1000 rpm, so it was turning "only" ~2750 rpm at 55 mph and ~3500 rpm at 70 mph, both still well below peak engine efficiency.

That's my story, and I'm sticking to it. But they don't make cars like that any more.
 

·
Registered
2018 3.6r Touring, Black, modifications: 255-55-zr18 Continental DWS06
Joined
·
346 Posts
I have an idea....


Sent from my Pixel 2 XL using Tapatalk
 

·
Registered
2020 OB Limited, Titanium & Magnetite Grays
Joined
·
586 Posts
Tire air pressure and aerodynamics are at the top of the list. Air pressure should be read early in the morning and before it is driven and when tires are cold. I add 1-2 psi over posted cold psi. There are "green" tires that have less rolling resistance and there are tires that have too much of rolling resistance; look up your tires to determine where they fall on rolling resistance. Anything, I mean ANYTHING that changes the airflow around the vehicle has the potential to change mpg at higher speeds.

Some less likely considerations.
  1. Use only Top-Tier gasoline. Not knowing what was used by the previous owner your vehicle could have some deposits that could be removed by using only Top-Tier. Your intake valves could be dirty if this is the case. Octane and brand of gasoline should not be a concern.
  2. Spark plugs are still new but who knows if...
  3. Sensors are never performing at the exact target setting. The O2 sensor could be replaced along with the mass air flow sensor. These belong on the list but are not likely a problem.
  4. Wheel alignment including the rear
  5. Exhaust system with some blockage.
  6. Find the last owner and ask them details about this vehicle.
 

·
Registered
2017 2.5i Premium Lapis Blue
Joined
·
3,974 Posts
There are "green" tires that have less rolling resistance and there are tires that have too much of rolling resistance; look up your tires to determine where they fall on rolling resistance.
Is there somewhere to even find information on rolling resistance of most tires? The only time I ever see it mentioned is with tires marketed as low rolling resistance tires. It would be nice to be able to easily compare rolling resistance of tires that don't fall into that category. For commercial truck tires that information is pretty much available for every tire but I don't see much for passenger vehicle tires.
 

·
Registered
Joined
·
3,357 Posts
Is there somewhere to even find information on rolling resistance of most tires? The only time I ever see it mentioned is with tires marketed as low rolling resistance tires. It would be nice to be able to easily compare rolling resistance of tires that don't fall into that category. For commercial truck tires that information is pretty much available for every tire but I don't see much for passenger vehicle tires.
I know CR gets a lot of hate but they do test rolling resistance for the few tires that they test.


Industry studies show that a 10 percent drop in rolling resistance equates to about a 1 percent improvement in fuel economy. It might not seem like a big difference, but fuel-economy gains are hard to come by. You can grab that advantage by making an informed tire purchase.
Our testing of performance all-season tires showed a 27 percent difference in rolling resistance between the best and worst performing tires.
Disclaimer: For me, fuel economy isn't top on my list and I never consult consumer reports in selecting a tire.
 

·
Registered
2005 Subaru Outback XT MT / 2012 Subaru Outback 2.5i Premium
Joined
·
53 Posts
You disregard the fact that while the cylinder dimensions of the H4 and H6 are very similar, the H6 has six of them. That means ~50% more friction loss (at the same rpm), ~50% more heat transfer loss (at the same power level), and ~50% higher pumping losses.
A big displacement four vs a smal displacement four would more obviously display the point I'm making. I still believe the data will show that the H4 at high load simply uses more fuel (BSFC) than the H6, because the RPM must be significantly higher, which leads to more thermal loss, more friction loss, but less pumping loss due to the large throttle opening. Your final drive calcs are ignoring most of the point since the 80 mph average is over varried terrain. I did record economy over flat areas like Texas, so that data would be more favorable to the H4. For areas of AZ, NM, Utah, and California, the terrain favors the H6. The best economy with the H6 is through hilly areas where the H4 is close to death trying to keep up.
 

·
Registered
2016 Outback Premium 2.5 CVT w/EyeSight+SRVD
Joined
·
6,927 Posts
For areas of AZ, NM, Utah, and California, the terrain favors the H6.
As a reality check, here's my anecdote for the day: a 272 mile day trip around north-central Arizona in our 2016 H4 Outback, August, 2018, at elevations ranging from ~3000 to ~7000 feet ... Winslow -> Payson -> Camp Verde -> Sedona -> Flagstaff -> Winslow:
489296

P.S. My anecdotes are just as valid as yours, neither more nor less.
 

·
Registered
2017 2.5i Premium Lapis Blue
Joined
·
3,974 Posts
As a counter-argument, here's my anecdote for the day: a 272 mile day trip around northern Arizona in our 2016 H4 Outback, August, 2018, at elevations ranging from ~3000 to ~7000 feet ... Winslow -> Payson -> Camp Verde -> Sedona -> Flagstaff -> Winslow:
View attachment 489296
P.S. My anecdotes are just as valid as yours, neither more nor less.
Last fall I was still showing over 40mpg after 61 miles. And that was after I had my dash MPG display adjusted down 6% by the dealer. I ended up with that tank ending up at 39.9mpg hand calculated, though I suspect the pump shut off early when I filled the tank. I believe both the dash display and my Scangauge II showed that tank a little above 37mpg and the Scangauge is usually pretty close to reality.

489297
 

·
Registered
2017 2.5i Premium Lapis Blue
Joined
·
3,974 Posts
Last fall I was still showing over 40mpg after 61 miles. And that was after I had my dash MPG display adjusted down 6% by the dealer.
Oops, I lied.
I noticed the odometer after I posted and realized that picture was from two years ago, so that wasn't taken on the same trip as my 39.9mpg tank. And that one was before I had the dash mpg display adjusted down 6%. Either way that was still pretty fun to see over 40mpg after an hour of driving. I tried going back to Fuelly to see what that tank ended up at but I didn't start using Fuelly until about 4 or 5 months after that pic.
 

·
Registered
Joined
·
8 Posts
I have owned a 2019 OB Touring 3.6R for about 15 months and 21K miles.
Average 27 mpg mostly highway in Maryland no traffic most drives. Delaware at 58MPH and very flat roads.. I have got 30mpg.
I just installed the K&N engine filter and switched to full synthetic oil 5w/30... first 3 oil changes were regular 3.6 factory oil. I must say I really like the 3.6R
My Xwife has had 4 Foresters.. 2.5 engine in her latest 2017 model and have seen it get 34mpg in Delaware.
 

·
Registered
2005 Subaru Outback XT MT / 2012 Subaru Outback 2.5i Premium
Joined
·
53 Posts
As a reality check, here's my anecdote for the day: a 272 mile day trip around north-central Arizona in our 2016 H4 Outback, August, 2018, at elevations ranging from ~3000 to ~7000 feet ... Winslow -> Payson -> Camp Verde -> Sedona -> Flagstaff -> Winslow:

P.S. My anecdotes are just as valid as yours, neither more nor less.
I haven't known the computer to display the correct fuel consumption. You should at least calculate it.
 

·
Registered
2016 Outback Premium 2.5 CVT w/EyeSight+SRVD
Joined
·
6,927 Posts
I haven't known the computer to display the correct fuel consumption. You should at least calculate it.
Your assumption is incorrect. I have long since had the fuel mileage constant in the Outback adjusted -6%, so the onboard display is consistently within +/-2% of hand calculated.

... Or you can go to Fuelly.com (just click on the Fuelly banner in my signature) and see the exact calculated fuel economy for that entire trip. There were several tankfulls (all segments generally above 4,000 feet in Arizona, New Mexico, and Colorado) that calculated from 35+ to 38+ mpg. In fact, the full 50,000 mile fuel history of my Outback is there for all to see.
 

·
Registered
Joined
·
400 Posts
Your assumption is incorrect. I have long since had the fuel mileage constant in the Outback adjusted -6%, so the onboard display is consistently within +/-2% of hand calculated.

... Or you can go to Fuelly.com (just click on the Fuelly banner in my signature) and see the exact calculated fuel economy for that entire trip. There were several tankfulls (all segments generally above 4,000 feet in Arizona, New Mexico, and Colorado) that calculated from 35+ to 38+ mpg. In fact, the full 50,000 mile fuel history of my Outback is there for all to see.
I don't suggest a human trusts a computer to double check a computer. Both have error. Hand calc is the only accurate way I know I can "trust". Simple math! All a computer CAN DO is "estimate".

Heck, I don't even know if the trip is "accurate"..... if I can't trust the cars computer for MPG.... wish we still had a spinning wire. (I know the detriment)
 

·
Registered
2016 Outback Premium 2.5 CVT w/EyeSight+SRVD
Joined
·
6,927 Posts
I don't suggest a human trusts a computer to double check a computer. Both have error.
Huh?

Hand calc is the only accurate way I know I can "trust". Simple math!
Are you asserting that using a pencil and paper is somehow more accurate than using a hand calculator or a computer?

"Trust, but verify." ~ Ronald Reagan
"Check your work." ~ every good teacher I ever had

All a computer CAN DO is "estimate".
The problem with Subaru's fuel mileage display isn't a computer problem, it's a data problem. It isn't an error in the calculations, it's uncertainty in the fuel delivery data input to those calculations, which is based on the long-term sum of fuel injector ON-time pulse widths. Consider that: 1) the fuel delivery per fuel injector pulse is measured in microliters, 2) fuel delivery vs. injector ON pulse width is a nonlinear function, 3) each fuel injector has its own slight variation in flow rate, and 4) there is some interaction among injectors sharing a common fuel rail that affects flow rates.
 
81 - 100 of 103 Posts
Top