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If I can shift it into neutral and coast using no RPM's that should make a difference from getting out of the gas but still turning 2,000 RPM's. Its the RPM's that really knock down the gas mileage
When you coast while in gear using engine braking you don't use any fuel as the injectors are shut off and the motor is kept turning by the momentum of the car. Since there is no combustion going on you get engine braking since the friction and force required to pump air in and out of the cylinders work to slow the pistons down. Coasting in neutral, the car needs to use gas to keep the motor turning instead of using the wheels to turn it so in theory you should use more gas.
 

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Any idea just how far in rpms the fuel stays shut off? Would a log with injector duty cycle show it?
I was thinking that the fuel gets shut off during coasting, but wasn't sure and didn't want to say something stupid here. I guess that'll happen anyways, sooner or later...
But as I'm often driving at low rpms (1500-2000), this is of interest.
 

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Discussion Starter #224
At the same time though... the compression slows the vehicle down, whereas idling in neutral doesn't... Just air resistance, and road/drivetrain resistance does.

There are times that I'll pop it in neutral for a downhill, that otherwise I would've lost speed by keeping it in gear and decelerating. I don't know which is better. It's probably a wash in the end.
 

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At the same time though... the compression slows the vehicle down, whereas idling in neutral doesn't... . It's probably a wash in the end.
Still, my commute home ends with a good downhill. To keep speed in check I roll down that hill in 2nd, at 30+mph and still have to hit the brakes some. I don't want to be speeding right there at home... The subie has not much compression braking, even my little 1600cc 4-cyl suzie has more... but there's other occasions where it gets more interesting. I have one long downhill that I can roll in 5th with hardly slowing at all, but very low rpms, like 1400... am I still saving gas here? or should I coast and burn the idle fuel? At some point it probably doesn't matter...
 

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Discussion Starter #226
IDK. If you have an accessport, you could probably figure that out.. but either way it's probably negligible.
 

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Friendly Neighborhood Spider-Admin
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I've always been led to believe that the amount of fuel burned at idle is tiny. I use coasting or engine braking on a situational basis. Also, sometimes I just want to hear the engine as I'm slowing down. :tongue:
 

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My MPG meter never showed more than 19mpg even driving on the highway at constant speed for 4 hours.
Only 1,500 on the car.
I will calculate it myself to see if the mpg meter is accurate.
But that is awfully low mpg.
 

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^ it is very low. I have a modded suby, it has larger injectors, therefore it shows way higher mpg's that is real (33 +/- in city). In reality I get around 20 mpg city, 28+ highway...
 

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My MPG meter never showed more than 19mpg even driving on the highway at constant speed for 4 hours.
Only 1,500 on the car.
I will calculate it myself to see if the mpg meter is accurate.
But that is awfully low mpg.
If this is a new car then this is normal, at least is what I was told when I got mine way back in 06. Once the car is broken in and reaches at least 3K then you start seeing better mpg.
 

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19mpg highway is normal, for a new WRX? Do you guys cruise at Mach 1? :tongue:
Seriously though, my hybrid build engine, when brand new, was ticking off 25mpg pretty regularly.

I got 27.5mpg on the way back from CretinFest, last month. :)
 

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If this is a new car then this is normal, at least is what I was told when I got mine way back in 06. Once the car is broken in and reaches at least 3K then you start seeing better mpg.
Really? Modern day engines and complete vehicles have very little break-in needs. 19mpg highway sounds plain wrong...
I suggest the old method: fill up to full, go most of the tank and fill up again. Compare miles gone / amount needed to fill tank again to full...
 

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Admiral Ackbar the 1st
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I think this documentation took a very long time to produce and it seems to contains valid experience from real drivers. Thanks for taking the time to maintain it.

I'd like to introduce myself. Call me Tommy. Other than knowing how to drive a car, I know nothing about cars. I figured this would be a good place to start learning.

I just purchased a 2012 Subaru WRX STi about two weeks ago. It is an amazing ride. My route to work and home is about ~100 miles a day. It's probably a little more than 800 miles on the odo now.

I've only been to the office 7 day these past two weeks, but I'm normally there 5 days a week. The images showed my current lifetime odo reading which is the same as the trip A reading and trip A's mpg reading.

Those readings are based on my driving style when I brought it at 10 miles...

98% Highway / 2% City - 8 out of 14 days
65% City / 35% Highway - 6 out of 14 days

Avg. % of Shifting less than 1.8k rpm - 2%
Avg. % of Shifting around 2.3k rpm - 18%
Avg. % of Shifting around 2.8k rpm - 20%
Avg. % of Shifting around 3.2k rpm - 50%
Avg. % of Shifting around 3.5k rpm - 6%
Avg .% of Shifting around 4.0k rpm - 3%
Avg .% of Shifting greater than 5.0k rpm - 1%

97% Intelligent mode
2% Sport Sharp / S# mode
1% Sport / S mode

Highway Cruising in 6th at around 2300rpm [50%] - 3100rpm [50%]
I have the rev set at 3200rpm which keeps me under 80mph.
I like to boost on the way to work. (5-10 second boosts, 2-3times a trip. 10 second+ boosts, 1-2 times a trip, 2-5 minute boosts, 1-2 times a trip)
I stay off the boost on the way home with minor exceptions. I usually boost uphill when the lane is clear.
I stay off the boost at least two miles before I reach my destination, and I leave the car idle for two or three minutes when I get to the destination.

It probably did 80mph for ~3 hours during it's life so far.
It probably did 90mph for ~15 minutes during it's life so far.
It probably did 90mph+ for ~3 minutes during it's life so far.
It probably did under 80mph for the rest during it's life so far.
Welcome to the club and thanks for posting up your information :)
 

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Friendly Neighborhood Spider-Admin
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The number of new-gen guys that are getting dreadful mileage is a little discouraging.
 

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Admiral Ackbar the 1st
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Nice data logging! I'm surprised you do as well as that at high altitude and with 91 octane

I know, another rather old post - it got me to ruminating though.

The octane effect at elevation is increased, so that might cancel out the effect. If timing was actually retarded it would have a negative effect.

Otherwise the altitude will have less oxygen, which will meter less because the ECU senses absolute pressure, and thereby cause a reduction in fueling - basically reducing the amount of power and the efficiency. Boost levels will drop relative to sea level. Having less power on tap will reduce ability to accelerate rapidly, increasing mileage if it overcomes the probably slight reduction in efficiency. Plus you have a significant reduction in drag. However, you have more hills. Some owners of other brands do report significantly higher mileage at high elevation in vehicles that can compensate fueling for the reduction in O2.
 

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My MPG meter never showed more than 19mpg even driving on the highway at constant speed for 4 hours.
Only 1,500 on the car.
I will calculate it myself to see if the mpg meter is accurate.
But that is awfully low mpg.
Just do it the old fashion way. Get a full tank, start the trip calculator. When you fill up, just figure out the difference.
 
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