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Old 03-06-2008, 07:12 PM   #1
ZeppelinPie792
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Default So my fuel consumption seems abnormally high

I was generally getting about 26-30 mpg in my 2.5i with 80% highway driving. After I had put on my borla headers and stromung high flow cat, I have only 179 miles with my tank being half full. Yes, I filled it to the top. My brother's WRX is at 260, just being barely below half.

Is it becuase I didn't reset the ECU (by disconnecting my battery and pressing the brakes a few times then reconnecting it)?

I also had just gotten new tires on at the beginning of the tank. They seem to grip the road better. I am not sure what kind they are, I just know they are called "high performance tires".

Also, I am due for an oil change, but it should have probably effected it before the past 179 miles.

I have no CEL's, and did not change my driving habbits. If anything, I am not stepping on the gas as much.

What is the problem?
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Last edited by ZeppelinPie792; 03-06-2008 at 07:21 PM.
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Old 03-06-2008, 07:33 PM   #2
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You reset your ecu?
Generally for the first 100 miles, after you do that. You dont get as much power, and less gas milage.
Drive it out, do your oil change.
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Old 03-06-2008, 07:42 PM   #3
iRick.
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Default

i have a question i don't get. why my car is completely stock

i got new high performance tires

i removed the snorkus

and suddenly my gas meter drops insanely quick

i'm getting gas almost once a week now
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Old 03-06-2008, 07:46 PM   #4
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tire pressure??
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Old 03-06-2008, 08:04 PM   #5
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It's the tires, a tire with more "grip" creates more friction at the road surface that the engine must overcome, therefore reducing your mileage. Same with going to wider tires. If you swapped some mileage friendly all seasons on there I bet your gas mileage would go back up.
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Old 03-06-2008, 08:11 PM   #6
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^^
or over-inflate your tires... but that will give you poor handling and increase wear-down rate
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Old 03-06-2008, 08:42 PM   #7
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-I didn't reset my ECU. I am going to change the steering wheel out within 3-5 days, and will just do that then.

-Taking out the snorkus did not effect my fuel economy a few tanks back

-My tire pressure is normal

-I heard that it was a myth on the over-inflating your tires part.

Modern steel belted radial tires will not deform their tread pattern under high pressures. Remember that tire is designed to be ran at its sidewall rating, so pumping it up that high will do no harm. Also you'll notice a significant increase in handling and steering response. Not sure if you've ever been to a track event, but while at the track most people pump their tires way up there to decrease sidewall flexing

The same principal holds true for high FE driving, except were increasing pressures to reduce rolling resistance. You'll notice a significant increase in the coasting distance in your car.

Those were taken off of a gas-mileage forum
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Old 03-07-2008, 01:31 AM   #8
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As mentioned in a few forums around here comparing miles per tank (or half tank) is pretty inaccurate. I'd be patient, go through a full tank, and average out your MPG to get the most accurate snapshot of the change you've notice. If you have the ability to log your ECU you could see what your ECU wants for fuel trims (both long term and short term). This is how I discovered my track pipe was causing long term fuel trims to run rich. (rear O2 issues) You can also get an instantaneous fuel economy readout through logging, although i don't have any hard knowledge as to the accuracy of this parameter I'd guess its close.

Over-inflating will reduce grip in wet/snow/mud conditions without a doubt in addition to a rougher ride. I run +4 PSI over what is imprinted on the door for the best compromise between grip/ride/rolling resistance. Keep in mind the max pressure imprinted on the tire is rated for max load. Running max pressure with less than full load is what you are doing (unless you've added alot of weight to your car). Although the issue of ideal tire pressure could be argued to death depending on ones preferences/priorities.

If you really wanted to test rolling resistance you could log your g/rev (or possibly even the instantaneous MPG) over a flat stretch of road with a constant speed (ideally same day - same conditions) using your old tires and the new ones and average the results. Although few of us have time to do that.

And PLEASE reset your ECU after a serious mod like new headers and a HFC. After it learns (advances IAM) your new setup you'll be good to get an accurate reading of your fuel mileage).
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Old 03-07-2008, 02:35 AM   #9
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Quote:
Originally Posted by LTracer View Post

And PLEASE reset your ECU after a serious mod like new headers and a HFC. After it learns (advances IAM) your new setup you'll be good to get an accurate reading of your fuel mileage).
I was going to reset it when I was doing my O2 sensor stuff, and flat out forgot. I would just do it now, but like I said, I am changing out my steering wheel soon, and will just do it then. I don't feel like looking for a non-adjustable wrench that fits my battery's nut right now.

I know I shouldn't judge my MPG by just miles on a tank, but usually it is 200+ at the half way mark.

My best guess was just my car running rich since I never reset the ECU. It did smell funky, but I always thought that would be from deleting my exhaust manifold cat and changing my cat-pipe cat out w/ a HFC

Thats a lot of cats in a paragraph...
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Old 03-07-2008, 03:01 AM   #10
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so its my tires?

i have yokohama advan s4

and the mileage factor is killing me
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Old 03-07-2008, 09:33 AM   #11
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the fact that you went with borla headers on a car that came with equal length from the factory could be hurting your performance more than helping. Also, I know it's true with most of the mods I do, such as the intake and cat-back that they do give me better gas mileage. This can only be detected on long highway stints though, because I didn't mod the car to stay out of the gas. Most people tend to drive slightly more aggressive after new mods which also hurts your mileage.
As others have said tires with more grip also hurt mileage since they generate more friction to overcome. Wider tires also produce more drag.

Also as was said before, miles per tank is not an accurate measure unless you are driving the car to absolute empty every single time. Reset your odometer when you get gas. At the next fill-up divide the gallons you put into the tank into the miles you drove since the last tank and there you have it, mpg.

Are you getting any CEL's with the header and high flow?
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Old 03-07-2008, 03:14 PM   #12
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No CEL's at all.
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Old 03-08-2008, 10:33 AM   #13
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Quote:
Originally Posted by ZeppelinPie792 View Post
I was generally getting about 26-30 mpg in my 2.5i with 80% highway driving. After I had put on my borla headers and stromung high flow cat, I have only 179 miles with my tank being half full. Yes, I filled it to the top. My brother's WRX is at 260, just being barely below half.

Is it becuase I didn't reset the ECU (by disconnecting my battery and pressing the brakes a few times then reconnecting it)?

I also had just gotten new tires on at the beginning of the tank. They seem to grip the road better. I am not sure what kind they are, I just know they are called "high performance tires".

Also, I am due for an oil change, but it should have probably effected it before the past 179 miles.

I have no CEL's, and did not change my driving habbits. If anything, I am not stepping on the gas as much.

What is the problem?
179 miles on half a tank?? prettyh damn good if u ask me.
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Old 03-08-2008, 10:40 AM   #14
salisburyv
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Quote:
Originally Posted by LTracer View Post
As mentioned in a few forums around here comparing miles per tank (or half tank) is pretty inaccurate. I'd be patient, go through a full tank, and average out your MPG to get the most accurate snapshot of the change you've notice. If you have the ability to log your ECU you could see what your ECU wants for fuel trims (both long term and short term). This is how I discovered my track pipe was causing long term fuel trims to run rich. (rear O2 issues) You can also get an instantaneous fuel economy readout through logging, although i don't have any hard knowledge as to the accuracy of this parameter I'd guess its close.

Over-inflating will reduce grip in wet/snow/mud conditions without a doubt in addition to a rougher ride. I run +4 PSI over what is imprinted on the door for the best compromise between grip/ride/rolling resistance. Keep in mind the max pressure imprinted on the tire is rated for max load. Running max pressure with less than full load is what you are doing (unless you've added alot of weight to your car). Although the issue of ideal tire pressure could be argued to death depending on ones preferences/priorities.

If you really wanted to test rolling resistance you could log your g/rev (or possibly even the instantaneous MPG) over a flat stretch of road with a constant speed (ideally same day - same conditions) using your old tires and the new ones and average the results. Although few of us have time to do that.

And PLEASE reset your ECU after a serious mod like new headers and a HFC. After it learns (advances IAM) your new setup you'll be good to get an accurate reading of your fuel mileage).



Excellent response!!

also keep in mind, the refineries have a different blend summer/winter. always harder for your engine to atomize the fuel in combustion chamber in winter. so expect 4-8mpg less in winter.
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Old 03-08-2008, 10:44 AM   #15
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Quote:
Originally Posted by Subyroo2.5 View Post
179 miles on half a tank?? prettyh damn good if u ask me.
ball park... 23.8 mpg. not bad in winter. based on a 15.9 gallon fuel tank and the 179 miles driven... math is fun!!
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Old 03-08-2008, 11:46 AM   #16
ZeppelinPie792
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I do not believe it is the winter blend, as it had just gone up from around 24 to 26 in mid January. Keep in mind, I live in Texas, so it doesn't get too cold over here.

My lowest tanks were around 24 in winter, though. They generally tend to be at least 26.
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Old 03-08-2008, 11:50 AM   #17
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lol I'm going through the same woes as you. I'm now getting roughly 65-75% of the distance per tank I used to get in the summer but it's not winter-related because it started in July/August. >_<
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Old 03-10-2008, 03:56 AM   #18
ZeppelinPie792
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Ok, so I just filled up. You are going to get a kick out of this.

24.99 mpg

Still doesn't seem as good as my other tanks, but isn't nearly as low as I had thought.
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Old 03-10-2008, 01:40 PM   #19
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Quote:
Originally Posted by ZeppelinPie792 View Post
Ok, so I just filled up. You are going to get a kick out of this.

24.99 mpg

Still doesn't seem as good as my other tanks, but isn't nearly as low as I had thought.
I always think I am getting worse millage then I really am.. I don't pay much attention now to the gauge except as an indicator to fill up at a certain level when it's getting low. The only way to really know is to calculate miles/ gallon after a full fill up. Even doing so after one fill up is not that accurate. an average of 4 or so fill ups would paint the clearest picture.
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Old 03-10-2008, 03:57 PM   #20
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Quote:
Originally Posted by SubyRSdriver View Post
I always think I am getting worse millage then I really am.. I don't pay much attention now to the gauge except as an indicator to fill up at a certain level when it's getting low. The only way to really know is to calculate miles/ gallon after a full fill up. Even doing so after one fill up is not that accurate. an average of 4 or so fill ups would paint the clearest picture.
I also have a scangauge II in my car. It helps you determine what kind of gas mileage you are getting, and you can use it to your advantage to get better gas mileage. If you are speeding up a tiny bit and getting say 23 mpg at that exact moment, you can let off the gas a bit to just coast and get 28 mpg.

The calibration on mine was screwed up for the last tank, so it said I was getting about 15-20 mpg per trip. I guess it made me a little paranoid, so I tried to match it up with how many miles I drove and how much fuel I had left.

If any of that didn't make sense, it is because I just woke up.
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Old 03-10-2008, 04:16 PM   #21
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You're lucky. I rarely make it 300 miles on a full tank and I over-inflate my tires and drive pretty conservatively. Don't know what the problem is at all.
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Old 03-10-2008, 04:44 PM   #22
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Quote:
Originally Posted by mclovin View Post
You're lucky. I rarely make it 300 miles on a full tank and I over-inflate my tires and drive pretty conservatively. Don't know what the problem is at all.
I get 600 km+ (372 miles) on 50% hwy 50% city driving on my 2.5i with modded WRX header, and 2.25" catback. I expect that to improve when I put my snorkus back on
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Old 03-10-2008, 04:52 PM   #23
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What am I doing wrong? I put on an STI catback a few months ago, but don't think I've reset the ECU ... other than that it's bone stock.
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Old 03-10-2008, 05:52 PM   #24
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Quote:
Originally Posted by mclovin View Post
What am I doing wrong? I put on an STI catback a few months ago, but don't think I've reset the ECU ... other than that it's bone stock.
City driving all the time? I get ~20mpg all the time doing city driving and/or occasional twisty fun. Car is bone stock.

P.S. Miles per tank is useless.

Last edited by ArcticWolf; 03-10-2008 at 05:57 PM.
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Old 03-10-2008, 05:56 PM   #25
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maybe you are just hammering on the gas because of the new headers.
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