Air Filter and gas mileage - it really matters!
 

I was recently noticing what I considered to be a drop in gas mileage on my P5. It wasn't horrible, as I do plenty of city and stop-and-go driving, but I felt it was dropping.

So at my last oil change (last weekend) I changed the air filter. This had already been done once before, but this one probably had 7-10k miles on it. It didn't look all that dirty, but since I bought the new one, I changed it anyway.

I must report that the difference is SUBSTANTIAL. On the tank before the swap, I got 274 miles on 10.77 gallons of gas - that's just about 25.5 mpg. After the swap, I hit 310 miles on 11.303 gallons, which equals 27.4 mpg. This is with very similar driving conditions - the test is reasonably accurate. Basically, I got about 20 extra miles out of 3/4 tank or so, and when you drive 300+ miles a week like me, it adds up quickly.

2 mpg is a good jump for just replacing the filter. I just might have to get an intake or at least a K&N stock replacement filter and see what a difference that makes.

And for those who wonder about the numbers - I have an auto tranny, and I'm reasonably nice to my car - I only gun it when I have to (or want to), but generally, I drive 70-75 mph in clear traffic. Plenty of city-type driving, too - and I carry about 100-150lbs of tools and crap in the hatch every day.

Change them filters often everyone!

~HH

Uhhh, this is a great topic.
Of course if you have a dirty filter gas mileage will go down .

What I am saying is that the filter MAY need replacement MORE often than recommended, and it may not look dirty even though it is.

Basically, it's an often-overlooked part in routine maintenance. And getting an extra 2 mpg by changing a filter is significant, especially when that filter is way less than 10,000 miles old, and doesn't look very dirty - plus, I just wanted to report some actual numbers showing the benefits.

But thanks for the sarcasm anyway.

~HH

Sorry dude just playing with you.
Yeah just get a K&N filter.

Sorry. Not a very strong data. Now if you said that you
averaged 25mpg for several months before the filter
change and averaged 27mpg after the change, I would
find that compelling. Two data points do not make a
trend.

I have checked the milage on my car every time I
fuel up since I purchased last December. Two,
even three MPG swings from tank to tank are
not that uncommon. Even under similar driving
conditions. This is based on years worth of data
on other vehicles.

My point is, if the filter looked clean, you probably
wasted your money replacing it only after 7K miles.

Yeah, I've taken alot of stats classes and I'm gonna have to agree with mecary1, you're data with a deviation of 2miles per gallon. Your experiment seems like it has some bias, and the correlation might only be minimal, especially if the margin of error would be a few MPG per tank. (considering the fact it only increased 2 mpg)

And in my experience as well and all the cars I have owned in the past the mpg did vary by a few mpg every time. It has to do with alot more factors than AF (which is a minimal factor), more with using AC, overdrive, Heat.... crap like that. And if your filter was clean... thats because it probably was. So.... thats my two cents +/- 2 cent error.

I can understand your skepticism of my numbers - I mean, they certainly aren't written in stone, and I could have taken a few more weeks of analysis to get it more accurate.

But my only defense is that for several weeks I noticed that my mileage wasn't what I thought it should be - that I was using gas faster than I though I should. So I did the math, and then swapped the filter.

I can say that the two tanks compared were VERY similar - nearly identical amounts of highway/city/stop & go driving, and similar weather (for the use of AC, heat, defrost, etc).

~HH