Dear Car Talk:

Our past two vehicles have been Toyota RAV4s, and we've always had the regular maintenance done every 5,000 miles. Although we don't drive on dirt roads, it seems as if we're shown air filters that need replacing more frequently than the maintenance book calls for -- which is every 30,000 miles or 36 months.

Are we being shown OUR air filter, is the condition of our air filter being exaggerated, or should we be replacing our engine air filter more frequently than every 30,000 miles or 36 months? Thanks.

-- Gary

Geez, Gary, if anything, I feel like air filters seem to last longer than ever these days. We see lots of air filters in the shop with 40,000 or 50,000 miles on them that are still perfectly serviceable. 

It's possible that your mechanic is showing you an air filter that may be fine, but may look dirty to the untrained eye (i.e., yours). 

As a layman, you probably can tell the difference between a brand-spanking-new air filter, just out of the box, and one that's not brand new. But you probably can't tell the difference between two used filters -- one that's still got useful life in it, and one that's as clogged as one of my brother's old cigar butts. 

I'm not sure why air filters seem to last longer these days. Certainly the air is cleaner overall. And in the old days, the crankcase ventilation system used to sometimes expel oily air close enough to the air filter to get sucked in. But that doesn't happen anymore.

If you want to do a test, let them change the filter next time, but ask for your old filter back. Then take it to another shop (check our Mechanics Files at mechanicsfiles.com for recommendations), and ask, "If this were your air filter, would you change it?"

My guess is the answer will be "no." Unless you're doing a re-enactment of Lewis and Clark in your RAV4, I'd be surprised if you need a new air filter more often than every 30,000 miles.