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Consumer reports did some testing on air filters a couple years ago, here's an excerpt:
"Change your air filter
The test team at Consumer Reports was sure that a car would get better mileage with a clean rather than a dirty air filter.
"It stands to reason that if the air is allowed to flow freely, it would result in better fuel economy," spokesman Douglas Love said.
To do the test, the team used duct tape to partially cover the air intake, simulating a dirty filter. And the result?
"We were surprised to find out it didn't much matter," Love said. "The mileage was about the same."
So, testers added a bit more tape and then some more. Short of blocking off all the air, they got the same results.
"We found that the onboard computers that adjust the fuel mixtures on recent cars did a surprisingly effective job," he said.
The auto club ran a test with clean and dirty fuel filters on several different cars. Results were similar.
"There was even one test where the mileage got a little bit better -- maybe 1% or 2% -- with the dirty filter," Mazor said.
But before you count the money you can save in air filters, there is a price to pay.
"The trade-off was that carbon monoxide emissions went up," he said."
TLDR: on fuel injected computer controlled cars, you're not going to see a FE difference in any filter, go for the best filtering capability to keep crap out of your engine.
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