Now that it's getting toward summer, which is less detrimental to fuel economy? Keeping the windows up and turning on the AC, or putting down the driver's side window (causing drag)?

And by AC I mean, the Auto climate control at around 70-68 deg.

 

There are a lot more variables to consider. One of them being speed. No A/C with windows down at 25 mph. No A/C and windows cracked at 50 mph. etc...

I don't recall if it was this forum or not, but it's been posted that it's ideal to drive with the A/C off and the windows cracked 3-4 inches. Best of both worlds.

But, a few cents per tank is well worth the cost for me. I'd rather ride around with the A/C on. Usually, in the mornings when it's still around 70 degrees, I'll drive to work with the A/C off and windows cracked. Anytime after about 10 am or 80 degrees I close the windows and set the climate control to 68.

 

Alone: all 4 windows open an inch and the fan on 4 or 5.
Wife in car: AC on max
kids, no wife: they don't have veto power yet.

The idea is that the slipstream sucks the air out of the cabin through the small opening of the windows, all 4 to avoid that beating noise, and the fan replaces it with new air. But the windows are not open enough to significantly affect turbulence and airflow detachment.

The whole 'which is worse- AC compressor or increased drag' argument can and will go on forever. Each car is a little different, though in general I lean towards saying that the windows all the way down yields worse FE than the AC, especially if you use RECIRC.

I do not use auto- I prefer to set the fan speed myself.

One thing I have never understood is why RECIRC is not the normal mode. Even then, something like 50% of the air is still fresh- cabin air is not a closed system by any means.

 

I've read that the MPG reduction with A/C on is 25%. Windows open, no A/C, probably 2-5%. As already said, speed matters. With the sophistication of the MPG displays on some of the hybrids, I'd think someone would come up with a real world difference at an average 55-60MPH and then again at a city speed of 30-40MPH. I've also read a hybrid gets dinged more than an average car. I've also read a rotary engine (like my RX-8) loses a lot w/ AC on. I generally don't use it a lot in my personal cars(compared to the family van) and would qualify as a 'hypermiler' in most cars besides my RX-8. I get 32-34 MPG in my 92 Miata, and the just-acquired 2001 Miata's first tank was 31.5MPG.

 

I stand corrected; AC definitely is a bigger hit to FE than windows down, though the magnitude of the hit depends on speed and engine load (faster speed/higher load, the incremental loading of the AC is less, resulting in proportionately less FE hit).

My apologies; I somehow got started thinking the other way around, and convinced myself that it was right.

The realworld numbers do exist:
From the Ford hybrid seminar, some hard numbers:

- 1.2 mpg loss using A/C vs. windows down at 60 mph.

- A/C off vs. on makes a dramatic difference. A/C on vs. off at 30 mph equates to just 30 mpg vs. 50 mpg or a 40% loss! At a highway speed of 75 mph, A/C on lead to ~ 25 mpg vs. ~ 27 mpg w/out or a 10% loss was displayed in the graph presented.

***note- the FEH is a very different animal- the massive loss at 30mph is because the engine has to come on a lot more than it would without the AC. Still, the point is that %FE hit depends on how much of a % load the AC represents to the engine at whatever point its operating at.

https://www.greenhybrid.com/learn/ar...ion.html/page2

 

AC on! ))

--
johntvery@mail.com

 

Even at very fast highway speeds, having the windows partway open uses up much less energy. Althoguh drag increases, it's not necessary to have the windows open as much the faster you go. When going around at 30mph or less, I'll have the windows all the way open, when going at more like 40-50, a bit less than halfway, and I only need them open a crack when going 65+ for the same effect.

A/C is still nice though if I want to cut down on the noise, or it's very hot out, in which case, it's best to run the AC on max when going down hills or braking, and on a lower-power setting (ie 80deg) the rest of the time.

 

I almost exclusively drive with the 4 windows open ~1inch. I may use AC for a short blast to quick-cool the interior, or if passengers are with me. But for my commute, no AC- as long as the slipstream keeps the wind going.

If its crawling and 95deg out, FE is the last thing on my mind, so the AC comes on then...