the trouble with the way science is taught in schools is that they don't teach it as something you do in your daily life to answer questions. like 'does AC cause more drag than opening the windows?'

well, i'm probably not that abnormal in the experience of having driven in a few cars with different size engines and air conditioners and windows (not all of them had air conditioners, they all had windows) and i've never been in a car where turning on the AC didn't cause an immediate and obvious load on the engine, and i've never been in a car where opening the windows had any noticeable effect on the engine/speed/power.

for example, take my current car, a (now aging) honda civic. small, aerodynamic, other side of the scale from the big flat SUVs you mention; if there were any significant aerodynamic effect from opening the windows this would be the car to see it. particularly at the speeds i drive.

on the other hand, the AC is quite puny. of course so is the engine, so it's very sensitive to increased loads.

well, turning on the puny AC is such an added load on the engine that I actually use it as a drag brake on a long steep downgrade during my daily commute. at idle, of course, you can detect the idle speed drop when you hit the AC button; despite the gadgets that crack open the throttle etc. to carry the load. not a huge load, mind you, but significant. that magic 10% plus or minus 5% that people always assign to such things.

on the other hand, even with this car and ungodly speeds, when i open all the windows at once quickly, thanks to the miracle of electric windows and a big hand, no effect at all on the engine note, the speed, anything; whether the sunroof is open or shut. my hand isn't big enough to reach the window switches and the sunroof switch at once.

but if i can't detect an effect of opening the windows on an aerodynamic car with a small engine at high speed, i doubt it's going to have an effect on an SUV with a V8 at 65.

as for the obviously more significant actual real world effect of using the AC on my gas mileage, comparing fillups on weeks when it's been AC all the time versus weeks when it's not: no difference immediately visible (i should mention that i'm enough of a nerd to have a spreadsheet which graphs the gas mileage for every fillup and i never miss updating it, so i would see a systematic drop if there were any) Maybe if i were to do a huge metaanalysis over all the years i owned the car i might seem some fraction of an MPG, but clearly, the variations from one fillup to the other in terms of how many errands i run in the city vs whether i drive an extra 20 miles on the highway are enough to swamp out any effect of the AC on mileage.

Really bad way to make the comparison. First, there are probably some startup costs with the A/C. Second (and most important), unless you have cruise set (and a very, very reactive cruise computer), the drag from the windows down isn't going to load your engine. It's going to slow you down. And rather slowly.

The reason you feel a load at idle, but not at high speed is not due to the A/C load being more than the drag load. It's that the engine's ouput is significantly higher at 3000-4000 rpm than at idle. Becasue idle power is so low (even in 99% of V8s) you have to give the car some gas when letting out the clutch (or to get the automatic tranny to transfer power to the wheels). At high rpm the engine is producing enough power so the load of the A/C or the drag of the open windows are a small percentage of the power the engine is producing - that 3-10% decrease in gas mileage w/A/C on means that 90-97% of the engine's output is going towards pushing the car through the air or accelerating. At idle, the A/C compressor load is a large percentage of the engine output (which is why the car's computer tells the throttle to open a bit more to adjust for the additional load of the A/C). Remember internal combustion engines have zero output at zero rpm and need the idle rpm setting just to keep running (essentially just enough to power all the needed equipment - A/C, alt., water pump, etc).

Thank you, Zarni. I was about to write the same post.

"I actually use it as a drag brake on a long steep downgrade during my daily commute."

How do you figure turning on your AC functions as a "drag brake" going down hill? Does it pop something out into the slipstream to increase wind resistance? Your transmission would slow you down, particularly if it is manual. Your brake definitely will. But, I don't see how running your air conditioner's compressor would effect it, at all. There's seems, to me, to be no connection between turning on the air conditioner and increasing aerodynamic drag and the force of gravity. You know, mass in motion and all that Newtonian physics stuff...

Now, if you are going uphill, that's an entirely different story.

Doc Holliday:

"I actually use it as a drag brake on a long steep downgrade during my daily commute." How do you figure turning on your AC functions as a "drag brake" going down hill? Does it pop something out into the slipstream to increase wind resistance? Your transmission would slow you down, particularly if it is manual. Your brake definitely will. But, I don't see how running your air conditioner's compressor would effect it, at all. There's seems, to me, to be no connection between turning on the air conditioner and increasing aerodynamic drag and the force of gravity. You know, mass in motion and all that Newtonian physics stuff... Now, if you are going uphill, that's an entirely different story.

because going downhill with the throttle closed and the AC off the car continues to accelerate, and with the AC on it does not continue to accelerate. nothing too complicated.

Gzuckier

You'd rather switch on your A/C to slow down your car than use the brakes?

Use a $700 compressor for something it was not designed to do verses a $19.99 set of brake pads.

What an idiot.

Gee officer, I didn't mean to run over her, I turned on my A/C to slow down!

"Use a $700 compressor for something it was not designed to do "??? here i thought you were allowed to turn the AC on some times. my mistake. we all know nothing extends the life of your compressor more than never turning it on.

"gee officer, i didn't mean to hit the back of that car, but my brakes were fading from using them down that long hill rather than destroy my AC by turning it on".

Maybe it's time to trade in the ol' Rambler and get a vehicle with a modern brake system. If your brakes are overheating from simpley going down a long hill, you have major problems and should not have to resort to turning on your A/C compressor to hopefully slow your car down. If you are going down a mountain each day on your commute, perhaps you should learn to downshift to slow down - that is a function your transmission is designed to do.

texyank:

Maybe it's time to trade in the ol' Rambler and get a vehicle with a modern brake system. If your brakes are overheating from simpley going down a long hill, you have major problems and should not have to resort to turning on your A/C compressor to hopefully slow your car down. If you are going down a mountain each day on your commute, perhaps you should learn to downshift to slow down - that is a function your transmission is designed to do.

gee, a couple of days ago you were warning me not to "Use a $700 compressor for something it was not designed to do [i.e. turn the AC on when coasting downhill] verses a $19.99 set of brake pads.", today you're advising me to downshift to slow down.

look, kid, find somewhere else to have an argument, I'm just not that interested.

Really?

Ask any mechanic, car dealer, parts professional (not an $8.00 hr kid at a parts chain store), or any rational person that has ever driven or even riden in a car, and they will tell you that it makes a lot more sense to use your brakes or a lower gear going down a long grade to slow your car than to think that it is better to try to use your A/C compressor to slow you.