"This tip also applies to warming up the car. Unless it's below freezing, cars don't need to be warmed up at all. Driving them gently is the best warm up there is. If it's 25 degrees out, you might want to let it warm up for 30 seconds. If it's 10 degrees out, warm it up for a minute. If it's -10 degrees out, move somewhere warmer."

Dear Tom and Ray: I live in San Jose, Calif., where the weather is almost always beautiful. The temperature rarely drops below freezing, even in the winter. However, when I walk my son to school in the morning (any morning, not just in the winter), I often see cars sitting in driveways running, without any driver or passengers. It seems like a waste of gas to me not to mention the fact that the cars in question are often huge SUVs used to drive a child two blocks to school. I also find myself wondering how often cars are stolen when people get in the habit of doing this. My question is: Why do people leave their cars running in the driveway for several minutes in the mornings? Does it serve some useful purpose? - Linda TOM: No, it doesn't, Linda. My first thought is that you shouldn't do anything. These knuckleheads are just wasting their money by burning expensive gasoline, reducing their already lousy mileage from 10 miles per gallon to nine and a half and slowly going broke. Don't interfere.

RAY: Well, that's a little harsh. You might take a kinder approach, Linda, and just move the car randomly to someone else's driveway. That'll mess with their heads.

TOM: Actually, they probably just don't know that warming up a car is completely unnecessary. Modern cars don't need to be warmed up at all even where there IS real winter. You start the car, you put it in gear, and if it goes and doesn't stall it's ready to go.

RAY: Right. Driving the car at a moderate speed, without hard acceleration, for a few minutes is the fastest way to warm up an engine. You don't want to jump right on the highway and floor it; three to five minutes of "in town" driving is all it needs.

TOM: If it's bitterly cold out, like 10 degrees, you might let the engine idle for 30 seconds or a minute, just to allow the oil to fully circulate and heat up a bit. But otherwise, warming up a car is a complete waste of time and gasoline.

RAY: Now, these people might be more concerned with heating up the passenger compartment than warming up the engine. In which case, there's not much you can do, except let them go broke.

TOM: But let's assume that these folks just don't have up-to-date information. In which case, leaving a copy of this article on their windshield might be a kind way of spreading the news.

Once a vehicle is running, the best way to warm it up is to drive it. Driving cuts warm-up times in half. With computer-controlled, fuel-injected engines, you need no more than 30 seconds of idling on winter days before driving away. Older cars should only warm for a couple of minutes. Anything more simply wastes fuel and increases emissions.

More than the engine needs to be warmed up - so do the wheel bearings, steering, suspension, transmission and tires, all of which can be done only when the vehicle is moving. Avoid high speeds and rapid acceleration for the first three miles or so. The goal is to bring the whole vehicle up to peak operating temperature as quickly as possible while maximizing fuel economy.

Idling is not an effective way to warm up your engine even in cold weather. The best way to do this is to drive the vehicle. No matter how efficient your car is, unnecessary idling wastes fuel, costs you money and pollutes the air.

Avoid idling your vehicle, in both summer and winter. Idling wastes fuel, gets you nowhere and produces unnecessary greenhouse gases. If you're going to be stopped for more than 30 seconds, except in traffic, turn off the engine. In winter, don't idle a cold engine for more than 30 seconds before driving away. (Older vehicles, however, may need more idling time when first started. In cold, winter conditions all vehicles may need more idling time to warm up and ensure the windshield is fully defogged. Be sure your vehicle is warmed enough to prevent stalling when you pull out.)