Once it was considered good to keep the car engine idling a minute or two following cold starts. Today, with modern technology, the opposite is true. An engine operating under road conditions will warm up faster and run more efficiently than the one that is idling. Idling just burns gas ( on average, about a gallon an hour). When you have a full tank of gas, park the car downhill. This will prevent any gas from coming out of the tank. Parking in areas of less or no sunlight helps prevent the gas from steaming that would occur if you parked in the hot sun. Your car will stay cooler, too, and that means less gas consuming work for the air conditioner once the engine is started. Stay away from wide-track tyres if you want top mileage (汽车耗费1加仑油所行驶的路程) . Narrow-track tyres produce less friction and thus less rolling resistance. The same effect is achieved by adding three to five pounds above recommended pressure to each tyre: while this won't noticeably affect you car's sliding quality, it will increase tyre life and gas mileage. Check tyre pressure often, especially when the weather turns cold. The difference between winter and summer tyre pressure can be as much as eight pounds. This could cost you two miles per gallon. The main purpose of the passage is to tell us______.