Some folks wonder when to change their air filter, some just follow a time schedule. For those that want the max out of their air filters (stock or aftermarket), this is a device I installed recently. I have them on everything I own. Have used them for years. It is called Filter Minder. I know that K&N sells them, but they are really pricey there. Ballpark $35. I bought mine thru Ryder Fleet Products for $21. Great product. It is common knowledge that air filters do their best job at keeping dirt out after time. They are less efficient at trapping dirt when new. The balance is knowing when the filter is at it's limit. This guage lets you know. When the filter is starting to restrict air flow, the gauge will show it and you can then change the filter. Will pay for itself very quickly.