From there
website.
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However, these experiences do not mean you will also experience a change in your mileage. We certainly understand why it is theoretically possible for a consumer to experience a mileage increase after installing a K&N air filter or intake system, however, we do not go so far as to make a general claim that our air filters and intake systems will provide an increase in mileage.
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I think that we need to keep in mind that the power setting that are used in driving for FE are very low and that large volumes of airflow is not required. I have experimented with aluminum restrictor plates placed just before the butterfly with a 1/2" hole. The top speed was 65 MPH WOT. FE stayed the same on level terrain but during 1/2 mile climb the FE increase due to the throttle setting required. When I increased the opening the FE advantage disappeared and with the original opening the car was really undriveable because of acceleration rates.
One thing about the hunt for FE is that all cars react differently to modifications even between 2 of the same model. It depends on a lot of uncontrollable factors. Even changing the oil when changing the air filter could show in increase in FE just do to the oil change. Tank to tank testing really is not an accurate way to test for a mileage increase. YMMV (Your mileage may very)