Originally Posted By: JOD
Originally Posted By: k1rod
Can an air filter really go 50K miles?
Absolutely. That said, how long it will last will depend quite a bit on your driving conditions. I really think your best best is to remove the guesswork out of it and install a restriction gauge. Otherwise, you really won't know if the filter is or is not in need of changing. They're cheap and easy to install, and if it saves you 1 or 2 unnecessary filter changes, it'll pay for itself.
+1 A clean filter isn't as efficient as a dirty one and a dirty one doesn't let any more dirt though. The possible exception is when restriction reaches a very high level and it may start to suck some thru (usually happens only to poorly constructed filters).
The only downside to a dirty filter is a potential loss of power at WOT, but since many (if not most) filters have a 40+ percent overage in airflow capability to account for a dirt load, a WOT power loss will not show up until the filter is 40 % plugged. Just cruising around at part throttle, you may not see any changes until the filter is 80 percent plugged.
A restriction gauge will tell you exactly the right time to change the filter so you get the most use out of it. The general rule of thumb is to replace a filter when the restriction increases 2.5 kPa (0.75" mercury) over the stock clean filter reading.
Changing it before it's really needed may give you a warm and fuzzy feeling, and that's OK, but please don't fool yourself (or others) into thinking it's beneficial to the car.