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At what speed does it pay to turn the a/c on? My experience is cracked windows use less fuel.

As mentioned in that other post, based on my own experiments for my own car 45mph seems to be limit for windows open but I can push it to 50mph with cracked windows, depending on wind direction and wind noise. Too loud = close the windows

I remember from Mythbusters #38 their cutoff point for window open/excess drag was also 45mph.

From my experience no matter what the speed is, the windows are better down than having the A/C on. At all times.

Depends on how old your car is and the size of the engine, though. The larger the engine, the smaller load relatively on the engine. Also, today's compressors are much more efficient than even ten years ago, and more so than twenty years ago.

I haven't used my A/C since the end of May when I had to drive 600 miles in 100 degree heat, and only after 350 miles of having my left arm sunburned to hell did I concede and crank the air. My left arm is still darker than my right arm today. :)

I've had the same experience as lamebums in my 4 banger Elantra. The hit on the engine from running the compressor outweighs the hit of an open window even at 65 MPH.

i dont have a sg to test, but i usually have my window open. if im alone on the interstate, itll b halfway r less at 55mph w/fan, but with passengers ill have my window all the way up because passengers = # of windows rolled ALLLLL the way down! it kills me! and its hard to hear the treble in the music...

AC? What's that? ;)

I usually will have the windows down under 30mph (depending upon wind, might not even do it then) -- otherwise I've got the windows up and the fan blowing vent air on me. I've not voluntarily fired up AC in 2 years. I'll do it for my wife and I did try it out on the way back from the WFEC for Laurie (in the Insight). The hit on highway numbers was less than I expected by fair margin, actually.

i dont have a sg to test, but i usually have my window open. if im alone on the interstate, itll b halfway r less at 55mph w/fan, but with passengers ill have my window all the way up because passengers = # of windows rolled ALLLLL the way down! it kills me! and its hard to hear the treble in the music...

I'll take having all the windows down over the A/C anyway. And if you can't hear the music over the road, crank it up further. :)

This is coming from a guy who's completely deaf in the right ear and believe me when I'm driving all I hear is the road. Erm...and the music over it. :)

Don't know if anyone else's car does this, but when I hit the engine kill button I have installed on my shifter, the fan kills with the engine. When I hit the start button on the shifter, the fan resumes along with the engine. It is also kind of nice because it doesn't sit there and drain my battery when I am sitting/coasting with the engine off!

As for running the A/C I won't do it unless the fam is in the car with me or else it has to be pretty dang hot and miserable!

I'll take having all the windows down over the A/C anyway. And if you can't hear the music over the road, crank it up further. :)

This is coming from a guy who's completely deaf in the right ear and believe me when I'm driving all I hear is the road. Erm...and the music over it. :)

lol well, trust me, its completely audible with two windows up, but its hard to hear with wind roar coming from all 4 windows lol, and i listen to my stereo where it is just below audible distortion. the bass makes it hard to hear the treble too, that probably stifles it more than the treble, but 2 12"s will do that lol

Depends on how old your car is and the size of the engine, though. The larger the engine, the smaller load relatively on the engine. Also, today's compressors are much more efficient than even ten years ago, and more so than twenty years ago.I don't know about the compressor improvements. But, I agree w/lamebums. I take a much bigger A/C FE hit on the 4cyl. vs. the 6cyl.

As I always like to state to these questions, there is a third option...vent.

Vent on its highest setting is still much lower power usage than the A/C, so the crossover point where you should use vent is going to be lower than A/C...downside, its rarely as cool as having the window open even just a crack.

I don't have much of a choice as I bought my 2000 Civic without AC :D The only time my windows go down is when I'm having a cigarette, and I'm quitting VERY soon, so then I won't have to roll my windows down at all :D On a terribly hot day I will roll down the drivers-side window in traffic, or an inch or two while I'm driving, but the passenger window is always up. I usually just use the vent while I'm driving.

I like to crack my window, and the right rear window. You get a good amount of airflow for very little opening. I usually roll mine down most of the way for the first mile, at slow speed, to get fresh cool air in.

AC is a killer, even at highway speeds.

I like to crack my window, and the right rear window. You get a good amount of airflow for very little opening. I usually roll mine down most of the way for the first mile, at slow speed, to get fresh cool air in.

AC is a killer, even at highway speeds.

+ 1 for this method ^^^. I get good airflow this way. I also have an electric fan, but I don't like it much because of where I had to mount it. I use the vent with fan on the really hot days. We are having an unusually cool August here this year. :woot::Banane35:

I don't think this matter much. I drove from Boston,MA to Buffalo, NY (470 miles) a few days ago with AC on low. Got 54.66MPG. Doing it again today from Buffalo to Boston. Probably not going to use the AC much since it's cooler today.

I'd love to see your results on the return trip, XyKo! :)

What speeds were you driving on the trip up?

The best solution is Tree Hugger's - no A/C in vehicle (no drive belt, no additional weight). Windows up keeps drag down (nimimum for whatever car you own), but may not be possible on a hot muggy day (especially when you first get in that sauna).

If you have A/C, there are times it is useful (like when you have passengers to consider). I can stand the heat better when it's my choice.

When you wimp out and must use your A/C, there are ways to use it so as to have the least effect on your MPGs (like turning it off when under load, and on when going down hill). Turn it off when the car is cooled down and the intake plenum is also cooler from the air passing through it.

I try to keep the A/C off and the windows down. I crack the windows when I get too hot, and yes, when I'm about to toast I've been known to turn on the A/C. But for argument's sake, A/C off and windows up is best.

I once drove my wife's old LTD on an 800 mile round trip. I had the windows up and the A/C on for the first half, and the windows down and the A/C off on the return trip. Did the 55 MPH speed limit both ways. Got the same MPG for both legs.

However, what I did not try, was to see how stellar the MPGs could have been if I'd had the windows up and no A/C. I don't doubt I'd have done much better.

If you have A/C and want to use it - go ahead. If you want your best MPGs, don't use it and keep your windows up (or maybe cracked a bit). 45 MPH seems to be a good rule of thumb for rolling them up (I don't have power windows).

One thing to mention is that your A/C system needs to be run for a few minutes each week, to keep the bushings and seals lubricated. Do this on the hottest day and kill two birds with one stone (what a terrible phrase, almost as bad as "skin the kitty").

I'd love to see your results on the return trip, XyKo! :)

What speeds were you driving on the trip up?


Going to leave in a few minutes. Speed was 60-75MPH and I draft behind trucks for 80% of the trip. Also did a 200+ miles trip yesterday to the Toronto Blue Jays and Yankees game. Toronto night traffic probably hurt my mileage. Going to post up that results tonight also. Need to refill for the back trip.

Also, how do I have the mileage and stuff in my signature?

Signature: http://www.cleanmpg.com/forums/showthread.php?t=5095

Please be careful drafting. Keep at least 3 seconds back - it's safer and legal that way.

Oh, I actually draft at a safe distance. During my trip to Buffalo, I drafted for safety precaution, that a few times other hypermiler cut right in front of me to steal the sweet spots.

Cool. I'm also eager to hear your comparison results.

Trip was not as I expected.

NY interstate I90 to Boston. Lots of high elevations and no A/C. Only drafted 30% of the trip, due to truckers going 75+ MPH. I was traveling 60-70MPH. Had to downshift and get out of lean burn to climbed the hills.

Distance: 399.7
Fuel used: 7.140

55.98 MPG

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Trip to Toronto for the day. No A/C. Was stucked on stop and go traffic for 5 miles on the QEW. Drove 100 miles on the NY interstate.

Distance: 316.5
Fuel: 5.992

52.82 MPG

Oh, I actually draft at a safe distance.

There is an oxymoron if I ever heard one. I don't believe you drive at a safe distance and get a useful draft. If you get a useful draft, you aren't driving safely...

Hmmm, sounds like something from Pink Floyd.