[the ambush]

Originally Posted by chaos311clarity

when you are rolling down the highway... your differentials have almost NO effect on anything... becuase you are moving in a straight line... therefore there is no added resistance and therefore no lost gas mileage

you said it: ALMOST no effect. and 21.6mpg is ALMOST 22.8mpg. but it`s not .

theoricaly: just as there will never be 2 dragsters with the exact same ET( if so just add another decimal until it differs). there is no way the front drivetrain will ever spin at the EXACT same speed as the rear.

[Crash1973]

For all of you doubting Thomas's, I have actually done some research on this. I mean real world research and not reading crap about what the DCCD does and doesn't do.

There are three situations that I have tested, Highway only, City only and a mix of the two.

Results of Highway Only (cruise control set at 75 MPH):

With DCCD in Auto: 22, 22, 23, 24, 23, 24 MPG
Avg: 23 MPG
With DCCD in Full Rear: 27, 26, 26, 26, 27 MPG (I have one more trip to Arizona next weekend to get one more set of numbers for this one)
Avg: 26.4 MPG

Results of City Only:

With DCCD in Auto: 22, 21, 21, 21, 19, 22 MPG
Avg: 21 MPG
With DCCD in Full Rear: 22, 22, 22, 22, 23, 22 MPG
Avg: 21.17 MPG

Results of Mix of City and Highway:

With DCCD in Auto: 21, 21, 18, 22, 21, 22 MPG
AVG: 20.8 MPG
With DCCD in Full Rear: 21, 21, 22, 22, 22, 22 MPG
Avg: 21.67 MPG

As you can see, in the city the difference is negligible. I also believe that the one 19 MPG and one 18 MPG are anomalies but since they were part of my stats for my experiment, I included them. On the highway is where I noticed a huge difference. I do not know why, every time the cruise control was set at 75. I only have 5 Highway trips with the DCCD set at Full Rear AND my cruise set at 75. The 6th time my MPG came out to 25 MPG BUT my cruise was set at 83 MPH. Conditions for all numbers were similar. Temp between 68 and 75 degrees (I did that on purpose. I figured if the temps were too different it might skew the results), speed was 75 on highway, a maximum of 55 in the city (average was 45 mph), fuel tank was filled ALL THE WAY on every fill up for the experiment. One difference though is that on the highway only trips with the DCCD in auto AND full rear, my wife was in the car too. The city only and mixed part was just me in the car.

[midnit]

You don't think the gas mileage might have changed instantly because you pressed the trip button next to the odometer, do you?

Because that DOES change the AV MPG, whereas when I was messing with my DCCD it didn't seem to change it much at all. Unless you DIDNT mean instantly, lol, in which case I haven't done as extensive testing as Crash has.

[Crash1973]

Originally Posted by midnit

lol, in which case I haven't done as extensive testing as Crash has.

LOL! I actually started out to prove that changing the DCCD did absolutely nothing (had to settle a debate with a co-worker) and was completely surprised at the results. I absolutely cannot explain why it makes any difference on the highway. If anyone else can explain it to me, feel free.

[solovus]

Originally Posted by Crash1973

For all of you doubting Thomas's, I have actually done some research on this. I mean real world research and not reading crap about what the DCCD does and doesn't do.

There are three situations that I have tested, Highway only, City only and a mix of the two.

Results of Highway Only (cruise control set at 75 MPH):

With DCCD in Auto: 22, 22, 23, 24, 23, 24 MPG
Avg: 23 MPG
With DCCD in Full Rear: 27, 26, 26, 26, 27 MPG (I have one more trip to Arizona next weekend to get one more set of numbers for this one)
Avg: 26.4 MPG

Results of City Only:

With DCCD in Auto: 22, 21, 21, 21, 19, 22 MPG
Avg: 21 MPG
With DCCD in Full Rear: 22, 22, 22, 22, 23, 22 MPG
Avg: 21.17 MPG

Results of Mix of City and Highway:

With DCCD in Auto: 21, 21, 18, 22, 21, 22 MPG
AVG: 20.8 MPG
With DCCD in Full Rear: 21, 21, 22, 22, 22, 22 MPG
Avg: 21.67 MPG

As you can see, in the city the difference is negligible. I also believe that the one 19 MPG and one 18 MPG are anomalies but since they were part of my stats for my experiment, I included them. On the highway is where I noticed a huge difference. I do not know why, every time the cruise control was set at 75. I only have 5 Highway trips with the DCCD set at Full Rear AND my cruise set at 75. The 6th time my MPG came out to 25 MPG BUT my cruise was set at 83 MPH. Conditions for all numbers were similar. Temp between 68 and 75 degrees (I did that on purpose. I figured if the temps were too different it might skew the results), speed was 75 on highway, a maximum of 55 in the city (average was 45 mph), fuel tank was filled ALL THE WAY on every fill up for the experiment. One difference though is that on the highway only trips with the DCCD in auto AND full rear, my wife was in the car too. The city only and mixed part was just me in the car.

Thank you for making me not crazy

[Crash1973]

Originally Posted by solovus

Thank you for making me not crazy

You are welcome.

[HPH]

Um, while this conversation has focused on the DCCD, it seems to me that the focus should be on the measurement of the mileage.

As I recall from my 05 (and from other Subarus), this "MPG" number displayed on the dash is not really the best way to calibrate this discussion for two reasons. First, it "computes" the mileage from the odometer in combination with the gas guage (that is, there's not a flow meter in the gas line or anything like that to monitor consumption), and the gas guage isn't really precise enough to do a computation to better than about 10%; and, second, the MPG number is an average of the past several (but I don't know how many) miles of driving, not an instantaneous value.

Therefore, if the MPG number changes every time the DCCD is switched, clearly there's a link of some sort between the two, but it probably doesn't have anything to do with mileage. HPH

[tshrey]

I would like Crash to respond, but based on the numbers he gave I'm assuming his mpg was calculated from mileage and how much gas he filled up with, not the mpg display.

Where are you getting the info about the display using the gas gauge? That would have to be the most inaccurate calculation ever put in a fuel injected vehicle - it doesn't need some kind of flow meter, it knows what the duty cycle of the fuel injectors are running at and as a result exactly how much fuel is being consumed.

I'm not surprised by any of this except people reporting getting in the 20's for city driving and high 20's for highway. I was thrilled when I finally averaged over 20 for my combined city/highway use...

[HPH]

...it knows what the duty cycle of the fuel injectors are running at and as a result exactly how much fuel is being consumed.

Really? If this is the case, then it constitutes an effective flow meter. I'm afraid I'll need to see more credible documentation before I'm persuaded. HPH

[serendipity]

There is so much misinformation in this thread.

MPG is based on injector flow, not the gas gauge.

It is not based on "the last X miles" - it is a cumulative total since reset. It's the total flow, based on injector flow, divided into the total mileage since reset.

If DCCD has a non-negligible impact on highway mileage, it's because the system is broken (whether by design or defect we can't say). DCCD in straight line cruise applies zero solenoid lockup. Look at the DCCD map.

Finally, there is no such thing as "full rear" and I wish people would stop parroting this piece of misleading nomenclature. It is full OPEN, and in the 06+ cars, it's not even all the way open; it's as open as it goes but there is still a mechanical LSD in the diff which can effect a torque transfer if there is slip.

[solovus]

I thought DCCD changed the torque distribution from 35/65 to 50/50 and that by setting it manually you force these distributions somehow. If this is the case then it could be caused by difference in power transfer losses between the front and rear...does this make any sense?

[snorky]

Originally Posted by HPH

Um, while this conversation has focused on the DCCD, it seems to me that the focus should be on the measurement of the mileage.

As I recall from my 05 (and from other Subarus), this "MPG" number displayed on the dash is not really the best way to calibrate this discussion for two reasons. First, it "computes" the mileage from the odometer in combination with the gas guage (that is, there's not a flow meter in the gas line or anything like that to monitor consumption), and the gas guage isn't really precise enough to do a computation to better than about 10%; and, second, the MPG number is an average of the past several (but I don't know how many) miles of driving, not an instantaneous value.

Therefore, if the MPG number changes every time the DCCD is switched, clearly there's a link of some sort between the two, but it probably doesn't have anything to do with mileage. HPH

Your 05 STi had a MPG display?

[chaos311clarity]

Originally Posted by solovus

I thought DCCD changed the torque distribution from 35/65 to 50/50 and that by setting it manually you force these distributions somehow. If this is the case then it could be caused by difference in power transfer losses between the front and rear...does this make any sense?

the torque distribution never changes... its always 35/65... what changes is the amount of lock in the center differential. thats it.

[HPH]

Your 05 STi had a MPG display?

Maybe not -- details such as this are hard for me to remember in cars past. Perhaps it was the Audi before that. HPH

[CGMDan]

The '05 does NOT have a MPG display.