Dear Tom and Ray:

My mother (age 75) bought a 1996 T-Bird with automatic Overdrive, and the O/D is the default when the car is started. Since she only drives around town and is afraid of the highways, the car rarely gets above 45 mph. The fourth gear (O/D) kicks in around 38 mph. So at these low speeds, the engine is only spinning at about 1,300 rpm.

To avoid gumming up the engine with carbon, I've been trying to get her into the habit of switching off the Overdrive (a button on the gear shifter) when she shifts out of Park. Am I correct? Since I am only her son, she doesn't believe me. But I think she will believe this advice when it comes from $25-an-hour professionals. -- Grant

RAY: Twenty-five dollars an hour? The only $25-an-hour professionals I know of nowadays are garbage men! Actually, I think they're up to $26.50.

TOM: Besides, I think you should just leave dear old Mom alone, Grant. She's not hurting anything by leaving the transmission in Overdrive.

RAY: The only time we tell people to turn Overdrive off is when the transmission is "hunting." Hunting is when it's shifting back and forth a lot and generally making a pain in the neck of itself (which it may do at certain speeds under certain driving conditions). But even then, we don't suggest turning off Overdrive for mechanical reasons; it's just so the constant shifting doesn't annoy the driver.

RAY: And I wouldn't worry too much about carbon. She probably drives so little and so gently that this engine is going to last forever anyway. So why make her life more complicated?

TOM: Wait! I just figured it out! Grant's worried about carbon buildup because he's hoping to inherit the T-Bird. That's why he's so concerned about how Mom drives it.

RAY: Right. I can see it now. She was all set to get a gray '96 Buick Century, and he told her that car has only been around for about 12 years, and they haven't worked all the bugs out yet. So he tried to talk her into Corvette. Eventually, they settled on a T-Bird.