Quote:
Originally Posted by tasdrouille
To elaborate on that, people usually change their air filters way too early. As air filters accumulate dirt, they also filter better. So do yourself a favor, get an air filter service indicator and only change your filter when you need to.
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They do filter better when dirty, because the pores get smaller. They are also more restrictive when dirty, hurting mpg. I am not familiar with an automotive filter service indicator, but where I work we deal with industrial filtering and use a differential pressure gauge spanning both sides of the filter to monitor filter restriction.