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rocket 04-29-2008 12:20 PM

windows vs. A/C
As the weather warms up, I'll need more than my vents to keep from frying while i drive. Is fuel economy better w/ the windows down (increased drag) or w/ A/C on (increased engine load)? I don't have the $$$ to get a scangage II yet to test myself. I drive a 97 ford aspire 5spd :turtle:. THANKS!

MetroMPG 04-29-2008 12:27 PM

In city driving, windows down generally trumps A/C use.

At higher speeds, the difference may depend more on the vehicle and the speeds you drive, I'd bet that for your vehicle, windows partly down is still best.

See this thread for more information:

DifferentPointofView 04-29-2008 11:29 PM

I took in account that I drive a brick, and big engines don't take such a big hit in MPG's from extra accesories because the amount of load put on it isn't very burdensome to the engine, whereas a small engine takes the burden more seriously.

So in the city, I drive with either vents on, or windows down.

On the highway, this is what I do. I don't see why more people don't do this, because it has shown me excellent results. Instead of running the a/c the whole time, I have the vents on, and when I get too hot, I press the A/C button, wait till the compressor has cycled twice, then turn off the a/c and use the vent again. The air remains cold for about 3 miles, and then goes back down to regular vent temps. I can usually stand it for about another mile, then i get hot (it's a good thing I'm cold all the time anyway :thumbup:) then I let it cycle on twice, and turn it off.

My drive is about 10-12 miles, so I only have to press the a/c button three times. a whole lot better than driving 10 miles with the A/C constantly cycling on and off.

I judge when it cycles on by the volt gauge, because when the volts go down about 1-2 volts, (from about 14) that's when the compressor is running.

When on the highway, I never have the windows down. brick+open windows=giant brick parachute.

Arminius 04-30-2008 01:14 AM

I do the same thing. Off, on, off, on.

boxchain 04-30-2008 01:18 AM

You can switch the AC on during in gear accelerations, taking advantage of DFCO. See:

Works best for stop/go type travels. I've found a 5-10s decel will cool the air down for about a minute. It's free and better than nothing :D

trikkonceptz 04-30-2008 06:52 PM

I started doing this also and quickly realized something .... it would be awesome if we could run a/c without the fan. In vent mode as mentioned the a/c remains cold for a while before it become hot, yet you can't activate the a/c without the fan.

They should invent an a/c mode that work with the fresh air vent so when cruising you shouldn't have to use the fan ...

boxchain 04-30-2008 08:44 PM


Originally Posted by trikkonceptz (Post 22479)
They should invent an a/c mode that work with the fresh air vent so when cruising you shouldn't have to use the fan ...

You don't want that, it's the same as running AC in your house with the windows open, you'll just pull in more hot humid air, making the AC work less effectively.

DifferentPointofView 04-30-2008 10:47 PM


You can switch the AC on during in gear accelerations, taking advantage of DFCO. See:
I've found that when I do that, it isn't good, the volts drop down, and when they do I have less pickup and use more gas. the Jeep also shakes from lack of sparking power. I get the best mpg's when I'm running almost 15 volts. and at night my dash lights are brighter :D

boxchain 05-01-2008 12:59 AM

Guh, I meant decelerations.

Harpo 05-01-2008 08:39 PM

Both work well
I got my windows tinted (FormulaOne Pinnacle 35) and it makes a bunch of diffence on how much you have to touch the compressor button. I still use almost exclusively DFCO AC because with my city driving. The DFCO ops are very common, as timing events for long Neutral coasting is few and far between. I use recirc in traffic just to keep the exhaust gases at bay and retain dehumidied air inside.

After the tint install, I've been using the Drivers window down more as the heat load in the car is far lower. I can fix the FF at 1.00gph, and rolldown /rollup the window, and see no change in mpg running at 50mph, my favorite speed. 70% of my return trip in the afternoon are country roads with shade and scenery. I hit the Sirius Blues Channel, roll the window down and set the FF. I'm home before I'm ready to get there!

I had a best on my return trip home today, at ~75 OAT, and average IATs around 105, window down, 24.9 miles, average 47mph, at 49.5mpg. Summer gas and calming blues, I reckon.

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