Question: While the air conditioner is on, would changing fan speed result in different fuel consumption? In other words, does the air-conditioner compressor run at a constant speed?--R.H.
Answer: The air-conditioner compressors on most cars have only one setting. There are many different types of air-conditioning systems, but typically the compressor cycles on and off, much the same way your home refrigerator does.
The amount of time that the compressor stays on depends on the engine speed, the outside temperature, the amount of cooling air that is entering the engine compartment and a lot of other factors.
On some cars, such as General Motors', the air-conditioning control has a normal and maximum setting. This affects only the amount of outside air that is mixed with the inside air. Maximum setting allows in 20% outside air, while the normal setting pulls in 80% outside air. But on some newer cars, notably some Toyotas, the compressor actually runs at two speeds, depending on the setting.
Your first question is more difficult to answer. The fan speed may have an indirect effect on fuel consumption. A higher fan speed would tend to put a higher demand on the air-conditioning system, which would theoretically force the compressor to be on more. While the compressor is working, the engine has a large additional load, which increases fuel consumption. I would say, however, that the effect of different fan speed settings on actual fuel economy would be negligible.
Q: I am the original owner of a 1974 Ford LTD with only 17,000 miles on it. The problem is starting it. It's OK if it is driven every day, but if it's driven only twice a week, it takes three attempts to start. Any solutions? --H.R.D.
A: The problem you describe is not unusual for many old cars. One possible cause is that the gas level in the float bowl of the carburetor drops over several days. Then a few moments of turning over the engine is needed to pump fuel into the float bowl to refill it. Unless you want to spend the money for a carburetor overhaul, you may want to live with the condition.
Q: Can you tell me if there are any four-cylinder cars on the market that don't have bucket seats? I hate to give up my bench seats, but I'll have to get rid of my old Toyota and buy a new car before long.--D.W.
A: It used to be that bench seats were standard equipment, but most manufacturers of subcompact and compact cars now offer buckets as standard.
Nonetheless, all three domestic car manufacturers offer four-cylinder cars with bench seats. The most familiar cars configured this way are the Chrysler Aries and Reliant. In Japanese cars, bench seats are generally not available.
Q: My owner's manual says when I rotate my tires I should not switch left and right tires. But I have had two tire professionals tell me that cross rotation is now acceptable. Who's correct?--S.M.
A: When radial tires were first introduced, it was believed that cross rotation could ruin the tire. Cross rotation changes the direction the tires turn on the road, and that once was thought to be harmful to the radial cord body.
Radial tires have been vastly improved through the years, and tire manufacturers understand the behavior of radials much better now. Most manufacturers no longer warn against cross rotation.
However, a good rule to follow in maintaining a car is to abide by the advice in your owner's manual. The engineers who wrote it may have had good reason for advising against cross rotation, not withstanding the improvement of the tires.