CLICK & CLACK'S CARTALK

LITTLE ROCK — Click & Clack (Tom and Ray Magliozzi) dispense advice about cars in Car Talk every Saturday. E-mail Click & Clack by visiting the Car Talk section of

www.cars.com

DEAR TOM AND RAY:

I often want to know in what direction I'm driving (plus I like gadgets). So I'll buy these stick-on-thedash compasses. They seem OK when outside the vehicle, but when mounted inside and in use, they give directions at times that I know are false. Do cars have a magnetic field of their own that interferes with the readings? Are the compasses too cheap? Is earth losing its polarity? Are you not the right people to ask?

- Paul

TOM:

All of the above, Paul.

RAY:

I think scientists have discovered that magnetic north is actually migrating. And they predict that in about 50,000 years, earth's poles are going to switch, so the North Pole will be at the South Pole. But I don't think your car will still be around then, Paul.

TOM:

The biggest source of electromagnetism in your car is your alternator, which is essentially a big electromagnet. But I would think that if it was throwing off your compass, it would throw it off all the time. You say "at times" you know the reading is false. Maybe that means at other times it's also false, but you just don't know it.

RAY:

Or it could be that as the output of the alternator increases with the speed of the engine, the magnetic interference increases.

TOM:

Any time a current is going through any wire, you're making magnetism. And there are lots of wires running behind and under the dashboard. So those could be throwing off your cheap little compass. But I'd say it's even more likely to be affected by things outside the car, like overhead power lines.

RAY:

So our answers are: Yes, there are sources of magnetism in the car. Yes, you're buying cheap compasses. Yes, we are not the right people to ask. If any of our readers have a better answer, drop us a note (you can e-mail us from cartalk.com).

DEAR TOM AND RAY:

My fatherin-law turns off his truck at every stop. Why? My wife tells me that he insists that he is saving gas. We live in Las Cruces, N.M., a town not associated with traffic congestion. You are not likely to sit idling for more than five minutes anywhere at anytime probably 95 percent of the time. Is this loony or what? What's the effect on miles per gallon; effect on the car; is this an inheritable personality defect?

- Matt

TOM:

Matt, this is obviously your first marriage. Because by the second or third marriage (take my word for this, Matt), every man learns that you should never try to get between your wife and her sainted father. That can only lead to trouble.

RAY:

Particularly when you happen to have your head up your keister, like you do in this case, Matt.

TOM:

Your father-in-law is doing a good thing for the planet.Idling is a complete waste of fuel. It increases our dependence on foreign oil, pollutes the atmosphere and puts wear and tear on your engine. Not to mention the squeeze it puts on your bank account.

RAY:

So turning off the engine when you're going to be sitting for more than a minute or two is a great thing to do. Most hybrid vehicles do this automatically. When you come to a stop, the engine shuts down, and the moment you touch the gas pedal again, it instantaneously starts up, so quickly that you don't even notice.

TOM:

And at least one energy expert we know suggested that if all vehicles shut down automatically at idle, we could cut our fuel consumption by 10 percent. Still sound loony to you?

RAY:

Some people are worried about turning off their air conditioner in the summer or their heater in the winter. But if you leave the fan blower running, there's enough residual hot or cool air to keep the car at a comfortable temperature while you sit there. Even in five minutes - which is much longer than the average person usually idles - the temperature inside the cabin won't change more than a few degrees.

TOM:

So here's your to-do list, Matt. First, start shutting off your engine when you're idling, especially if your wife's in the car. Next, tell your wife that you feel lucky to have married into such a smart family. After that, buy your father-in-law a nice bottle of single malt, sit down with him over a ball game, and ask him if he has any other tips for you.

RAY:

And finally, whatever those tips are, don't write a letter in the newspaper calling them loony. Good luck.