My dad says that using your car’s air conditioning saves gas when compared to opening a window(s). I understand that opening a window disrupts the vehicle’s airflow, but the compressor, electrical system needed for the fan, etc. require more engine power (and gasoline). Which is more energy efficient? 
Well, according to the Master here, it is pretty much a toss up.
FWIW, that article dates to 1986. The compressors used for AC systems are much more efficient and less likely to drag on the engine and affect mileage. For example, in my old car I could definitely see a difference in performance dependent on the AC’s status. In the new car, there is no difference at all.
This is a WAG, but I wouldn’t be surprised if more recent cars are more affected aerodynamically by windows being down. Current models are much more dependent on aerodynamics for their mileage ratings.
The power draw of the electric fan used to help cool the AC’s condenser is irrelevant. Increased power draw from the vehicles electrical system is not going to cause the alternator to increase its drag on the engine.
I would strongly disagree, the refrigiant used today is nowhere near the efficency of the old R-12, requireing more power to opperate. Also todays cars are not as aerodynamic as those comming out in the 80’s which were based on the fuel efficent cars at the end of the 70’s. Today cars are bigger (ever hear of a SUV?) and the engines are much more powerful, so runing an accesory like a a/c is a much smaller %'age of total power and much less noticable. I would also WAG that the cars computer will compensate for the a/c as well by upping power.
The old rule used to be for good ol’ R-12 speeds above about 45 mph the a/c is more efficient, below that speed the windows would be. Or to put it another way, when you are driving fast enough to get a good breaze, close the windows, when you are hardly moving to get a good breeze, open the windows and sweat :rolleyes:
Unquestionably it does also, power is not free and the more you use the more you have to replace, up to the limit that the alt can put out. If power from the alt is not needed it never gets produced and the load is reduced. You can (well could back in the old v-belt days) see this for yourself if your alt belt was not tightened enough, increasing electrical demand could cause the alt belt slip under the load.
A comparison sample size of one based on a subjective experence that doesn’t even involve the same vehicle is not valid. AFAIK, there was no general revolution in compressor efficiencies past the early 1980’s, and as kanicbird said the whole R12-R134a switch changes the equation completely such that it’s comparing apples and plantains.
No, it will, although the effect is perceivably small. Think on this - where else does the power come from? A car is not a perpetual motion device, and unless regenerative braking is doing the work, something ultimately generates that power…
Vehicles made today are much more aerodynamic than those of the 80s.
The advances in vehicle mileage in the 70s and early 80s had virtually nothing to do with aerodynamics and advances in engine design. Gains in mileage during this period can pretty much be linked to automakers simply de-tuning existing engines or dumping larger engines in favor of smaller ones.
Large cars and SUVs have existed for a long time. They are not some recent development. If anything, the fact that a vehicle is larger can substantially cancel out any power gained simply by using a larger engine. Further canceling out the gains of the larger engine is the fact that a larger vehicle will require a larger AC system to effectively cool its interior, and other systems working off of the engine have to be larger and take up more power as well. Also, just because a vehicle is larger in volume does not mean that it’s less aerodynamic. I think it’s a pretty safe bet that a SUV from the 80s will not be as aerodynamic as a current one.
Yes, R-134a is not as efficient a coolant as R-12, requiring a larger capacity system for the same cooling. However, compressor design for vehicles has advanced so that much more is accomplished with much less effort. Any advantage an older R-12 system might have due to it’s smaller size is pretty much negated, if not bettered, by such advances
You’re correct here. I meant to qualify that statement by saying “…cause the alternator to noticably increase its drag…” That’s what I get for writing while tired.
FWIW, I test-drove a number of cars a year ago when I was in the market. I can’t recall ever feeling a difference in performance dependent on the AC status. In all of the older cars I’ve driven I could.
I’d agree that general compressor design probably hasn’t advanced much in the last 20-30 years. What was a good design then is probably just as good now. This is probably also true for other components that draw off of the engine, such as the alternator, water pump, etc.
However, the design and implementation of such components actually used in automobiles has advanced substantially in this time period. Manufacturers have been looking at and, cost allowing, taking every possible advantage that they can take in order to maximize mileage.
I always do both. I figure HOT air blowing around the car is one thing; COOL air blowing around the car is another. It’s the best of both worlds to have warm wind in your hair and cool air on your face. Plus it has the added advantage of freaking people out: you’d be surprised what an entire shock it is to most people, the very IDEA of having the windows open while the airconditioner is on! Most people think it’s the most decadent thing they’ve ever heard of. It’s like watching a baby take its first taste of ice cream.
The Car Talk Guys answered this question back in 1994, and came down on the side of AC as more efficient in modern cars than lowering the window:
They don’t give a cite for the aforementioned “studies”, but I presume that this is something the folks in Detroit and Tokyo have been looking into quite somewhat.
lissener: *I always do both. […] Plus it has the added advantage of freaking people out: you’d be surprised what an entire shock it is to most people, the very IDEA of having the windows open while the airconditioner is on! *
Well, considering that it’s apparently the most fuel-inefficient way possible to cool your car (actually, I’ve gotta be wrong about that, the most fuel-inefficient way would probably involve running the AC with windows down and lugging several hundredweight of ice blocks around in the back seat, or something), if your passengers care at all about not wasting gasoline, you can see why they’d be shocked.
However, if the difference it makes to your comfort is worth the loss of a few mpg to you, 'at’s your decision.
I think a lot of it depends on the shape of the vehicle and if were talking about city mileage or highway mileage.
The guy in the boxy ups truck making deliveries downtown should probably have his windows open. The guy in the Taurus driving down the interstate should probably have the air on.