So, the two most popular personalities in public radio have announced their retirement.

Garrison Keillor and Scott Simon? Nina Totenberg and Sylvia Poggioli? Robert Siegel and Ira Glass?

Nope. Tom and Ray Magliozzi, hosts of “Car Talk,” will stop recording original shows this fall after celebrating their 25th anniversary on NPR.

“Car Talk” is the adorable little goofball of a show that draws huge ratings and has turned Saturday mornings (“Weekend Edition Saturday,” “Car Talk,” “Wait, Wait, Don’t Tell Me”) into the highest-rated and most lucrative daypart of the entire week for public radio. Many stations double-up on “Car Talk” during fund drives, broadcasting at least two hours every Saturday and sometimes another hour or two somewhere else in the weekend schedule. They would often schedule big raffles and challenge grants for Saturday morning because they knew they were reaching their maximum audience. The Car Guys produced special material for fundraisers, offered special premiums, and recorded promotional spots to pay elsewhere in stations’ fund drives.

The car guys will live on — in rerun material from their extensive archives, recut into “new” shows by the “Car Talk” staff. We’ll see how long they can keep that up; sooner or later, it’ll become noticeable that they never talk about recent-model cars. Eventually, the show will decline and disappear, although it may take years for that to happen. I’m sure the vast majority of stations will be happy to continue carrying the reruns as long as they can squeeze any value out of them.  

It’s ironic that “Car Talk” has been such a cornerstone of public radio’s financial world for so long, because it’s helped build a public radio system that’s too big and risk-averse to ever produce another show like it. These days, everything in public radio is tightly formatted and designed not to give any listener a reason to change stations. The days are gone when a couple of untrained ex-auto mechanics could become network stars.

As the shows have been homogenized, so have the individual voices. Unless you’re a hard-core public radio junkie, can you tell Steve Inskeep from David Green from Guy Raz from John Ydstie? When you tune into “All Things Considered,” do you know whether Michelle Norris or Melissa Block is co-hosting? No, and that’s the way management likes it. That’s why Bob Edwards was exiled to satellite radio; they didn’t want any single person to be bigger than the system.

Problem is, it’s hard for a “system” to engage the kind of passion that turns listeners into donors. That’s what “Car Talk” does, and “A Prairie Home Companion” does. And with “Car Talk” edging into the shadows, public radio will have to find new ways to induce the kind of generosity it’s now dependent on.