Can you please explain what "overdrive" is and what purpose it serves?

Dear Car Talk

Dear Car Talk | May 01, 1997

Dear Tom and Ray:

On my 1991 Camry, there's a button called "overdrive." Can you please
explain to me its functions and usage? -- Mike

TOM: Yes, Mike. That's the mother-in-law ejection seat. But it only works
once, Mike, so make sure you wait until she's in the middle of a good, loud
rant before you hit the button.

RAY: Actually, overdrive is just another name for fourth gear on your car.
Overdrive is a gear designed for high-speed driving. It provides little
power to accelerate, but allows the engine to run relatively slowly so you
get excellent gas mileage.

TOM: And that button is there to turn overdrive OFF. Now, at this point,
you may be asking yourself, "If I have an automatic transmission which
chooses my gears for me automatically, why would I ever want to turn
overdrive off?" That's a very good question, Mike! And in most cases, you
don't want to turn it off.

RAY: There's only one exception, really. There may be certain road or
traffic conditions that force you to maintain a speed that keeps the
transmission right on the hairy edge between third gear and overdrive. That
speed is generally about 35 mph. And at that speed, the transmission tends
to "hunt." That means it goes back and forth between third and overdrive
trying to find the right gear to be in.

RAY: Is that bad for the car? Not particularly. But it can be annoying to
the driver and passengers. And that's why that overdrive button is there.
So when the transmission is "hunting," you can turn off overdrive and force
the transmission to stay in third gear until your driving conditions

TOM: Other than that situation, I would suggest you just ignore the
overdrive button, as you've obviously done quite successfully since 1991,

* * *
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