Q Dear Tom and Ray:

Whenever I am driving, my dear husband and I have an argument about fifth gear. I was taught not to use fifth gear unless I am going to be driving on a long stretch of road without traffic lights or I am on the freeway (we have a 1998 Volkswagen Passat). My husband says it doesn't matter how long you're going to be in fifth gear; he says it's all a matter of how fast you are going. He says I got a simplified explanation because I'm a girl. He goes from first gear to second to fifth driving around town, even though there may be a stoplight a quarter-mile away. He says that he rarely uses third and fourth gears (which I use all the time in the city) unless he is hauling a load. Ease my troubled mind: Is this causing stress on our transmission? -- Mary

A TOM: Let's start with a freshman seminar: Fifth Gear 101.

RAY: The first thing you need to know is that it's easier to keep a car moving than it is to get a car moving. In other words, getting a car from 0 mph to 50 mph takes a lot more effort than keeping it going at 50 once it's already there. Newton figured that out after he invented the fruit-filled cookie.

TOM: So the gears in a car are designed for various degrees of difficulty. First gear, for instance, is designed for the hardest work, when you're moving a car from a dead stop or climbing a steep hill. In first gear, the engine's crankshaft may turn 100 times for every rotation of the wheels.

RAY: If you've ridden a multispeed bicycle, it's very similar. You know that in first gear, your legs (like the car's engine) pump the pedals many times, even while the bicycle wheels turn very slowly. But the pedaling is very easy on your legs. That's like first gear in a car.

TOM: And like high gear on a bicycle, your car's fifth gear is designed for the easiest duty, when you're already at speed and just need enough power to keep the car moving at that speed. So in fifth gear the engine just lopes along, turning the crankshaft only, say, 25 times for every rotation of the wheels. And that saves gasoline.

RAY: So, when should you use fifth gear? As often as possible -- as long as you're not "lugging the engine."

TOM: By "lugging," we mean that the engine is struggling to accelerate. How would you know it's struggling? Because when you step on the gas to accelerate, it doesn't go faster. It may bog down, it may buck, it may lurch. And you may get noises from the engine, like pinging or knocking. If any of those things is happening, you're going too slowly to be in fifth gear, and you need to downshift.

RAY: So, to address the dispute with "dear husband," Mary, it turns out you're both a little bit wrong. It makes no difference how long you plan to stay in fifth gear; the engine or transmission couldn't care less. So that's not a criterion for when to shift.

TOM: But "dear husband" is also off-base when he says that speed is the only factor. After all, you can shift into fifth at a lower speed while going down a hill than you can on a flat road. Speed is a good guideline, but the bottom line is, if the engine isn't complaining, you're good to go.

Dear Tom and Ray:

We are a couple of old broads in our seventies, both very hard-headed. My friend drives with her purse between her stomach and the steering wheel. I think this is probably very dangerous! I tried to tell her that when the air bag is deployed, it comes out with a great deal of force -- enough to push her purse into her ribs or stomach and do major damage. Am I right to worry about my best bud, or am I just a worrywart? -- Ouida

TOM: We checked with the air-bag experts at the National Highway Traffic Safety Administration, and they gave the purse position a thumbs-down. They're concerned that it could interfere with the proper path of the air bag.

RAY: So I suppose, instead of heading directly for her chest, the purse could deflect the air bag upward, sending the force of it directly into her schnozola. Or the air bag could break the pocketbook, allowing a piece of it to cause her a nasty injury. And then there's the factor of what's in the bag. I know that old ladies, by regulation, are required to carry large quantities of facial tissues. But she probably also carries some hard objects, such as keys, a compact or even knitting needles.

TOM: So the best place for the bag is on the floor on the passenger side, or right next to her, between the front seats. Tell her if she has an emergency need for a tissue or a picture of her grandkids, you'll be right there to provide an immediate assist, Ouida.

Got a question about cars? Write to Click & Clack in care of The Post, or e-mail them by visiting the Car Talk Web site at www.cartalk.com.

(c)2005 by Tom and Ray Magliozzi

and Doug Berman