Dear Tom and Ray:

After driving for 40 years and never having blown an engine or transmission, my husband blew up on a recent road trip (in which I was driving), because after two hours, he noticed that I still had the car in drive, not overdrive. He proceeded to read me the riot act. I always thought overdrive was for hilly or mountainous areas. Now I'm a nervous wreck driving anywhere farther than my own small town. Please tell me the rules of overdrive! I drive a '93 Buick LeSabre. -- Nancy

RAY: When I first read your letter, Nancy, I saw the line about your husband blowing up, and thought this was going to be about spontaneous husband combustion. My wife got very excited by that idea. But now I understand that you were saying he blew up, as in got angry.

TOM: Well, start by telling him to calm down. First of all, it's not nice to yell at your beloved wife for any reason. Second, it's only a car. In fact, it's only a '93 LeSabre. And most important, you've made the car last 14 years, Nancy. You're obviously driving it gently and doing a lot of things right. So there's no need for any riot acts to be read to anybody.

RAY: That said, you've got the overdrive thing completely backward, Nancy. Overdrive is, essentially, the highest gear. So, if you have a three-speed automatic transmission with overdrive, "overdrive" is fourth gear. Setting the shifter to overdrive means that you're allowing the transmission to use any of those four gears, right up through overdrive.

TOM: When you put the shifter in "drive," you keep the transmission from shifting any higher than third gear. So you're preventing it from going into overdrive.

RAY: There's really no good reason to limit the transmission to third gear. The more time you spend in your highest gear (overdrive), the better your gas mileage. And the higher the gear, the fewer times the engine turns for the same amount of distance traveled, so the longer the engine lasts. So, basically, Nancy, you want to put the shifter in overdrive all the time.

TOM: You're fortunate in that you have an automatic transmission. That means it figures out what gear it needs to be in, how? Automatically! So just let it do its job.

RAY: There are a few rare situations -- normally hilly roads -- when you might want to be out of overdrive. One is when you're going 35-45 miles per hour -- where the transmission will be right on the edge between third gear and overdrive. In that situation, the transmission might "hunt," going back and forth between those gears trying to find the right one. Some people find that annoying. So in a situation like that, it's fine to shift down into drive temporarily to stop it from hunting. But it's not necessary.

TOM: And when you're going down a long, steep grade, it's good to downshift to a lower gear -- third, second or even first -- so that engine braking keeps your speed under control, and you don't overheat your brakes.

RAY: But other than that, the best thing to do is just put it in overdrive the moment you leave your driveway, and then forget all about it.