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I installed the K&N replacement air filter on my 2008 Bullitt yesterday. I was averaging 18 miles to the gallon before the swap. Right after the swap I reset the numbers (I do this once a month). I logged in 100 miles on the new filter and average at 13.5-14 if I drive normally without too much revving. Is this a known issue? Should I just put the stock paper filter on? Does the Bullitt need more time to figure out its getting more air and needs to adjust the fuel supply?
 

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I've never heard of it being an issue, if anything the mpgs get better, a one week trial is hardly evidence of lower mpgs
 

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I agree with "2manystangs"! I have had a k&n drop in on my 5.0 for about 44,000 miles, and didn't notice any difference in mpg's from the first 15,000 miles on the factory paper filter.

I didn't buy the k&n for better fuel efficiency or claimed gains, I purchased it because it was only $20 more then the OE replacement paper filter, and because it is reuseable, and as you can see at almost 60,000 miles I would have almost purchased my 3rd paper filter a cost of $35 each vs $55 1x cost
 

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Discussion Starter #4
I agree with "2manystangs"! I have had a k&n drop in on my 5.0 for about 44,000 miles, and didn't notice any difference in mpg's from the first 15,000 miles on the factory paper filter.

I didn't buy the k&n for better fuel efficiency or claimed gains, I purchased it because it was only $20 more then the OE replacement paper filter, and because it is reuseable, and as you can see at almost 60,000 miles I would have almost purchased my 3rd paper filter a cost of $35 each vs $55 1x cost
Should have waited for a few days. Its good now. Guess the ECU needed a few miles to adjust. I love the sound after the install. My old ford racing filter was pretty clogged up after only 30,000 miles.
 

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Interesting... I put a CAI on mine and tuned it and my mpg dropped. Hopefully mine will do what yours did. I've only driven it twice since then (in the last week).
 

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Not a “Power Adder” thread. Moved to “Tech”.:bigthumbsup
 

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MPG varies more due to driving conditions and habits than anything else . . . if I drive down my street in first gear, I am going to get really bad mileage no matter what CAI is on my car . . . and right after we do new mods, we tend to get heavy on the throttle to try them out.

Short tests are really bad because there is not enough mileage for the different driving conditions to average out, or for you to calm down on the throttle . . . give it some time . . .

The only other thing that came to mind is that if the oiled filters are over-oiled, that can contaminate the mass air flow sensor and mess up the air/fuel mix.
 

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I have a K&N CAI on 05 GT and pretty heavy custom tune. I lost about 2 MPG.....
 

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mpg

I have run a K&N filter on my 2005 Gt for over 120,000 with no problems. In fact in my case the mileage picked up not dropped off. This was my wife's daily driver, she drove it 50 miles a day and averaged between 25-28 mpg for 6 years. One thing to always be careful of is re-tuning the car not to many tunes will ever increase mileage they are more about performance. Key to K&N filters is proper amount of oil that's where the fine filtering comes in. I've noticed on the forums here the repeated negative info from the diesel crowd on the K&N filters, if you spend a little time with one of their techs you would know that they recommend an additional filter over their regular filter in heavy abrasive environments. Last time I checked I haven't seen a diesel engine under many mustangs. It just goes back to proper prep equals quality results.
 
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