'Car Talk': windows up for mileage Window ruins her hairdo

Dear Tom and Ray: Please answer a question that has been bothering me for over five years. No one has given me a logical answer. I want to cool the passenger compartment of my car as efficiently as possible. Do you get better gas mileage (all factors being equal) by using the "2/60 method" (two windows rolled down, driving 60 mph) or by using the car's air conditioner?

RAY: Great question! As a matter of fact, the Society of Automotive Engineers has wasted some of its valuable time studying this very dilemma. The answer is that you get better mileage by rolling up the windows and using the air conditioner.


TOM: The reason has to do with aerodynamics. The more aerodynamic your car is, the less wind resistance it creates. Wind resistance makes the engine work harder and therefore wastes fuel.

RAY: Anyway, when your windows are all rolled up, wind slides around your car. That's when it's most aerodynamic. When you roll your windows down, wind comes inside the car, blows against the seats, the passengers and the back window and generally makes the engine work harder.


TOM: And according to the studies, you waste more energy through the loss of aerodynamics than you do through powering an air conditioner.

Dear Tom and Ray: I own a 1986 Volvo 240. I am pleased with my car, but I'd like my next car to have more pep. I am considering the Volvo 740 GLE with the multivalve engine. What do you think?

TOM: We were going to suggest you step all the way up to the six-cylinder Volvo 760, but we're afraid that might provide too much excitement for you. We'll save that suggestion for the next time you write to us. That'll be in about four years when you tell us you want something with more pep than your 1990 740 GLE.

Dear Tom and Ray: Here's one for you! Ever since my brother-in-law adjusted the idle speed on my '85 Mazda 626, it just hasn't been the same. Now when I turn on the air conditioning, the idle revs up and down, up and down, constantly. When I have the air conditioning off, it revs every time I turn the steering wheel at a stop. What can I do?

RAY: First of all, we're sending a copy of your letter to the Society for the Defense of In-laws. Everybody wants to blame his or her in-laws for something, and we've had enough of it! Your poor brother-in-law had nothing to do with your erratic idle.

TOM: He probably did clean out all the change from under your seats and spill cream soda all over your carpet, but he didn't mess up the idle.

RAY: Your car has what's called an idle-up diaphragm. When you use the air conditioner or power steering, you impose a heavy load on the engine. To keep the engine from stalling, a vacuum is automatically sent to the idle-up diaphragm to increase the idle speed. When the diaphragm is sucked in by the vacuum, it pulls a rod attached to the throttle, which ups the idle speed.

TOM: Your idle-up diaphragm probably has a small hole in it. So the vacuum can suck it in, but can't hold it in place. And as the diaphragm slips back and forth, the idle speed moves with it.


RAY: You need a new idle-up diaphragm. See if you can get your brother-in-law to pay for it.

Dear Tom and Ray: My husband and I have a 1990 Buick. The problem is that he simply refuses to use the air conditioner. We have had new cars for the past several years, and he always says that using the air conditioning in city driving will cause the car to overheat and vapor lock. We have had temperatures approaching 100 this summer, yet he insists we drive with the windows open, blowing hot air into the car and completely destroying my hairdo. I asked him if maybe cars these days are made so one could use the air conditioning without a problem. He says no. Please comment on this in your column.

RAY: Assuming that the cooling system on your car is in good working order -- and you would certainly assume that of a brand-new car -- driving in city traffic with the air conditioner on should be no problem. Trust us. If air conditioners could only be used in cold weather, they'd sell a lot fewer of them.