Air Cleaner - Gas Mileage - EFI



So, I made three changes to my car, and didn’t really expect any of them to affect gas mileage: plugs, air filter, less restrictive air intake.

Since it ran fine, before and after plugs, and the electronic fuel injections should always keep the air/fuel mixture the same.

I don’t understand how these changes gave me a 10% better MPG. Does anybody have any ideas?

BTW, car is a Acura TL 2004, manual trans.


What do you mean by a less restrictive air intake? Putting in an air filter that allows more air flow will result in less filtration. You might get better fuel economy, but you might pay a price in engine life. A dirty air filter will lead to poor fuel economy. The computer won’t keep the air/fuel mixture constant if you have a congested air filter. With a dirty air filter it will run rich.


The only change that could do this is the new spark plugs.


A car may run fine, but may have been in need of a tuneup. I’ve seen cases where I changed the plugs on a car that seemed to be running fine…but after I changed them the car ran a lot better.


I would need to know how you calculated your fuel mileage before I really got into this.

I disagree that the computer will not keep fuel-air mixture ?right? with a dirty air filter. It won?t keep it constant; it is not supposed to keep it constant anyway. A very dirty air filter could cause significant losses in power by use of more energy to get more air through it, but not because it goofs up the mixture.

How did the plugs look? I agree that this is likely the most significant factor unless limited testing was done and it was done in a windy place.


Good question. I did the same things, but used different plugs than OEM. Used Bosch Plat+2 plugs, K&N air filter, & IMCO peformance muffler. Gas mileage showed minor improvement (less than your 10%) but the throttle response is incredible, now. I’m now beating lots of cars so that probably negatively affects good mpg, I would guess.


When you are cruising down the highway at the speed limit, what is the biggest restriction to airflow? Why, the throttle butterfly valve that is only about 10% open of course. Compared to that restriction, a dirty air filter is almost totally unnoticeable.
Assuming that your closed loop EFI keeps the mixture correct, you should not see a change in gas mileage. The engine doesn’t care if a dirty air filter, a more closed throttle, or the high altitude of Colorado is reducing its power. The free-er flowing the air filter is, the more the throttle has to be closed to compensate if you are trying to maintain a certain power output.


I would suggest removing that K&N. Those oiled filters can cause expensive damage to your car if you put a little too much oil on it or don’t get it properly distributed. That in turn can damage the MAF.

In theory a less restrictive air filter can increase top speed, but in modern cars (like most cars built since the 70’s) it already breaths will with the stock filter. For less than wide open throttle, there is no gain! The throttle works by restricting the air flow in gasoline cars.


I would suggest the placebo effect. After all these things you are probably driving a little different and paying closer attention to mileage. You can’t really know mileage on less than 1000 miles of monitoring. There are just too many errors.


Opera has the right answer but this is the one that posters never want to hear. So let’s attribute the perceived improvement to new spark plugs and we congratulate the OP for performing timely maintenance.


I have definitely observed large gas mileage increases when I changed off a dirty air filter. I would recommend a stock paper air filter changed more often than the book calls for. Not a low-restriction one like a K&N which may pass dirt.


Spark plugs will do that in a gas engine. You get kind of lulled to sleep over time with a gas burner as the plugs get worn. The air filter, I kind of doubt did that much to help mileage. On my diesel, it definitely will help, but not as much on a gas engine.



Manners, please. You haven’t shown courtesy to acknowledge any of the respondents. Maybe you have passed out at the keyboard and can’t get up? Then I guess you’d have a good excuse.