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Old 12-08-2007, 11:00 AM   #1
Nitrous
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Default Fuel Economy Issues

Hey all, I've owned my '07 Impreza 2.5i since late June, but I'm starting to get worried since I'm paying attention to this issue now. It seems my fuel economy is now-- awful! I remember first getting the car and hitting well near 400 miles at a full tank, and now I can barely muster out 200 with an even more toned down style of driving. If I'm getting on it, I'll get around 150.

The only thing I've changed out is the axleback, springs, and wheels/tires -the OEM STi axleback. I also bought used '06 OEM STi springs, and the two front springs were a bit funky and took a really long time to set on my car, but I'd have more reason to believe it's my axleback. About that, I've noticed every once in a while I have to clean the tip, it gets dark and smoggy especially at the bottom of the ring. I had thought this problem was firstly because I had just installed my springs and as a result was getting bad fuel economy so I went in for an alignment and had everything adjusted back to stock. It seems I have little or no improvement over my milage, compared to what it was.

I would think this normal, except I'm not consistently redlining my car or anything. In daily driving it won't hit much over 3k rpms, as I shift around there. I haven't heard of N/A imprezas getting this kind of fuel economy either. I've heard only of STi's/WRX's driven into the ground to get 200 miles out of the tank.

Does anyone else have this problem, or am I alone here?


Thanks for any help, it would be appreciated.
WL
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Old 12-08-2007, 11:06 AM   #2
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how many miles on the car?


some of this could be attributed to "winter gas"


In my 2.5 RS (2001) I get approx 20mpg
and i drive harder than most.....

fuel economy does drop in the winter due to changes in the gas itself.


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Old 12-08-2007, 11:15 AM   #3
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^^^ Would explain why my mpg has gone down a bit in recent weeks...
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Old 12-08-2007, 11:19 AM   #4
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My car is now just a tiny bit over 5,200 miles.

I understand what you're saying with the 20 MPG, but the thing is I'm not even driving it hard. I'm almost afraid to get on the throttle. In daily driving I won't usually go far past 3k. I know for a fact if I let them climb for 5500-6200 for even just a tiny bit, I can tell the difference in the gas needle dropping down a tad bit.

Even if I got 20 MPG I'd be getting 338 miles to the tank with the 16.9 gallon tank. Either my simple mods have done something to hurt fuel economy, or winter gas REALLY is that bad!!

I just don't think getting between 150-200 miles per tank is reasonable in my car. :/

Last edited by Nitrous; 12-08-2007 at 11:29 AM.
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Old 12-08-2007, 11:31 AM   #5
Subie Gal
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winter gas "is" that bad I am afraid.
more ethanol than summer gas... not so great for MPG


there are other factors too, especially in colder climates
oil is thicker,
battery = harder to crank over,
the entire car is working working harder to keep going


@ 5,200 miles I've got to conclude that the season itself is the bulk of your issue
the mods you've done will not have an effect on this type of thing

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Old 12-08-2007, 01:08 PM   #6
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You said you changed wheels and tires!
What size tires are they? If their rolling diameter is different from the stock tires, that will directly effect your "indicated fuel milage" even though your real fuel milage may not have changed much. Do you idle the engine a lot to let it warm up when cold?

Double check your tire pressures low tire pressures can suck up a bunch of fuel. As Jamie said lots of reasons we lose fuel milage in the cool seasons other than the fuel. One of them is colder tires are stiffer and have more rolling resistance especially if under inflated so they have to flex a lot.

Are you still running OEM oil thickness? If you have changed to a heavier weight oil that could also be eating some fuel. There is a remote chance that something you did when you changed out the exhaust is causing problems. Are all your O2 sensors connected, if something got pulled off, like a vacuum line during your upgrades the ECU might be in limp mode -- I assume you have no CEL's recently.

Does the engine still warm up normally, a thermostat that is stuck open can over cool the engine and keep it in warm up mode all the time. Take a long slow look at all your hoses and sensors under the hood to be sure they are all still properly connected, sometimes doing very simple things you can accidently pull a hose, or connector loose.

The other possibility is that that muffler on that axle back is clogged up somehow, like a bad back fire broke something loose.

One of our local guys was having terrible performance and fuel milage for no apparent reason and it turned out to be a collapsed flex joint in an after market exhaust up pipe. A clogged exhaust system can really jack up your fuel consumption by strangling the engine.

Something is pretty obviously not right to take that big of a milage hit. A quick compression check would eliminate the possibility that the engine has mechanical problems.

If you are measureing your fuel milage by miles per tank, take the time to actually figure it out exactly by recording gallons used and miles on the odometer. That will give a much better comparison to work from. You could also try shifting to a different fuel brand or station on the off chance they miss fueled a ground tank.



Larry
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Old 12-08-2007, 05:48 PM   #7
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Yeah that's some serious MPG loss. Sounds to me like a failed component, not low tires, winter gas, etc.
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Old 12-08-2007, 05:50 PM   #8
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I've never had any CEL's go off. At 3k mi I changed my oil with Royal Purple, the rating that my oil cap specified. The wheels are nothing special and probably not the cause, they're OEM '06 WRX wheels with the stocker RE92's on them. I run 31 PSI all around on the tires, just a random number I came up with. I've read that this size wheel does not have a larger rolling diameter, so no changes are necessary-- but there would if I had used a 18" wheel.

I've also tried all sorts of gas. I usually go to Delta Sonic since it's cheaper by a couple cents but a few weeks ago I've went around trying different stuff, Hess, Mobil, etc, etc. They all seem to yield similar performance.

I played under the hood once, so I'll have to check all hoses and such. A K&N air filter was installed at the same time I got the axleback on.

The engine warms up just fine. Upon startup it idles at 1,500 and goes down to 500 when somewhat warm.
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Old 12-08-2007, 07:24 PM   #9
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200 miles on a tank (assuming you are getting about 13 gallons a fillup) is too low. That is what I get with a stage 2 STi that I beat on. Only 15 mpg!
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Old 12-08-2007, 07:25 PM   #10
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i dont think winter gas can kill your MPG by half...

I get about 260 on a full tank usually, but when winter got close.. I started getting 240ish. (all city, usually revving 4k max)
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Old 12-08-2007, 11:56 PM   #11
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All hoses and sensors look in place... I've still got no idea what's going on!!
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Old 12-09-2007, 12:30 AM   #12
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Double check your K & N filter is properly installed. It is easy to not have the lid latch properly on the air box (at least on the early WRX), if that sort of thing is possible on your intake it could seriously confuse the MAF due to the huge air leak and turbulence in the intake. That could make it over fuel trying to protect the engine.

Also try cleaning the MAF sensor, there are a couple threads from folks that tell how to do it. If it is contaminated with oil and dirt (which can happen if the K & N is over oiled) it can throw off the MAF signal and lead to mixture problems.

Larry
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Old 12-09-2007, 01:50 AM   #13
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Aren't the 2006 stock WRX rims 17 inch vs. the 16 inch stockers on the RS?
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Old 12-09-2007, 03:45 PM   #14
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Quote:
Originally Posted by sdogg3kgt View Post
Aren't the 2006 stock WRX rims 17 inch vs. the 16 inch stockers on the RS?
you are correct.
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Old 12-09-2007, 04:11 PM   #15
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damn, my sti under spirited driving gets better mpg.
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Old 12-09-2007, 04:40 PM   #16
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did you check the tank area for fuel leaks?
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Old 12-09-2007, 06:34 PM   #17
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Quote:
Originally Posted by Nitrous View Post
My car is now just a tiny bit over 5,200 miles.

I understand what you're saying with the 20 MPG, but the thing is I'm not even driving it hard. I'm almost afraid to get on the throttle. In daily driving I won't usually go far past 3k. I know for a fact if I let them climb for 5500-6200 for even just a tiny bit, I can tell the difference in the gas needle dropping down a tad bit.

Even if I got 20 MPG I'd be getting 338 miles to the tank with the 16.9 gallon tank. Either my simple mods have done something to hurt fuel economy, or winter gas REALLY is that bad!!

I just don't think getting between 150-200 miles per tank is reasonable in my car. :/


But you're not running the tank dry.

How many gallons are you putting in when you fill up?

And I'm doubting you were getting 400/tank without that being all highway...
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Old 12-09-2007, 07:43 PM   #18
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I miscalculated, the next time I fill up I'm going to write everything down and see.

Also on my error, I think the tank is 15.9 gal, as opposed to 16.9
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Old 12-13-2007, 06:57 PM   #19
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I always fill up to full + reset speedo, the other day I filled up 11.766 gallons ( 1/8 left of a tank from what I could see) . I always reset odo after filling up so I can assume that from 11.766 gallons I got 186 miles - roughly 16 MPG...

As I said I reset my speedo every fill up so I'll see how much this 11.766 gallons gives me to make my math more accurate. In any case -- this milage sucks, compared to what I used to get.

Either way, getting that type of milage around 11-12 gallons is pretty bad. I know for a fact I had once hit 362 miles running it near empty, driving the same way I am now. (gas light was on for a good day... drove ~25 mi that day)

It was some time after I had put on the STi springs, that I know-- but I can't see how that would effect my fuel economy.
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Old 12-13-2007, 07:04 PM   #20
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bring your car in for service ! if you are indeed getting that low of mileage out of a tank there is something seriously wrong
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Old 12-13-2007, 07:23 PM   #21
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Yeah. I'm going in on Monday
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Old 12-13-2007, 07:51 PM   #22
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If you have the defogger mode set on all the time like me, you will be chewing up gas by having the air conditioner on all the time. It sucks but it's better than foggy windows.
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Old 12-13-2007, 08:11 PM   #23
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That's low for this tire. Subaru recommends 33 psi front and 32 psi back for the 2006 WRX on stock RE92. You should probably try it. You'll most likely pick up some MPGs.
I personally started running 40 psi front and 35 psi back.

Quote:
Originally Posted by Nitrous View Post
[...]I run 31 PSI all around on the tires, just a random number I came up with. [...]
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Old 12-13-2007, 08:13 PM   #24
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Quote:
Originally Posted by mugenXP View Post
i dont think winter gas can kill your MPG by half...

I get about 260 on a full tank usually, but when winter got close.. I started getting 240ish. (all city, usually revving 4k max)
try livin in north dakota and drive on our gas...you will notice a huge difference
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Old 12-13-2007, 09:50 PM   #25
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Originally Posted by REX8 View Post


But you're not running the tank dry.

How many gallons are you putting in when you fill up?

And I'm doubting you were getting 400/tank without that being all highway...

I consistantly get 380-420 miles on a tank of gas. (normally 13-14 gallons at a fill up) The best I have ever gotten was coming back home from a weekend get away and I took the back roads so that I could stay out of boost.(so I am not running 70-75mph on the ineterstate) I set cruise control on 59 mph and drove almost 320 miles and only put 9.87 gallons in my car. Which gave me almost 32.5 MPG....I thought that was great gas mileage for a WRX. My mods are very simmilar to the OP's. Ct back exhaust and an intake.
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