open windows or not

I'm no automotive engineer, I just play one on TV...but I've read enough posts on here and read the manual to know that you get better gas mileage with the windows up AND if you use A/C, set the temperature higher. I usually go with 77 or 78. Still cooler than the outside air of 97 here in MO because of....well, you know, global warming.

Windows up, A/C set to 78. On Auto. Because the A/C will slow down once you hit and internal temperature of 78 degrees.

 

Yes, there have been many posts on this. However, there is no simple answer, because the difference is so slight, and the conditions are never the same.

I have found through my own testing (I drive around 2800 miles/month), that if I drive with only the drivers window down, and the drivers rear window cracked around 2-3 inches, I get the best mileage (by best I mean of the two choices, AC versus windows down). I normally get around 51-54mpg this way...and I drive this way almost all of the time.

However, now that the weather is hotter, and I am using the AC more, I only average around 48-49mpg with the AC on (set to LO with temp at LO on RECIRC).

I have tested it both ways on the same 35 mile commute home, at the same temperature and it is pretty consistent...I get around 3-4mpg less with the AC on.

Now, with all 4 windows down, that is a different story...there is definitely more drag...and although I have yet to test that for a long stretch (too noisy), it appears that it is around the same as the AC on.

Although the MPG's are better with the window down, sometimes it is just too hot (20+ days 100 degree plus here!)

 

<div class='quotetop'>QUOTE(speech path @ Jul 26 2006, 08:17 PM) [snapback]292965[/snapback]</div>

I don't know about that, but I'd say that in circumstances where you DON'T need AC or defrosting at all, I wouldn't turn auto or the defroster on so that there's no chance the compressor comes on.

Regarding whether driving w/the windows open or windows closed w/AC is more efficient, Mythbusters tested this twice, but not in a Prius. See http://www.answers.com/topic/mythbusters-season-2 episode 22 and http://www.answers.com/topic/mythbusters-season-3 episode 38.

 

The key point from the mythbusters episodes (even though it wasn't done in a prius), is that there is some point where the drag caused by the open windows will equal the MPG hit by running the AC. That specific point is probably different for all cars (depending on shape, etc). Based on experience, i'd think that the point is actually lower in the Prius than in other cars (such as the one they tested). It does make you wonder, though, what the point would be for only cracking the windows (1-2 inches)... i would think you could go faster that way than with them fully down.

 

I personally recommend you do what you need or want in with AC or windows. It's great if you can save a couple of MPG, but I sometimes feel that I worry too much about MPG after buying this car looking at my MFD every second. Our second car is 94 honda civic and when I drive that car, I don't care if windows cracked down or AC that much.

I like cool air of early morning here in KS, so I roll my windows all the way down, but when it gets hot, I wouldn't worry about my MPG and just turn my AC on.

AC in Prius is pretty good i think, and there's no really reason to leave it as 'Auto'. For me, low is more than enough.

 

My limited experience is that at highway speed (55+) using the AC set to auto and 77 is MORE efficient (or at least as efficient) then having the window down. I prefer to have the AC on at highway speeds (I don't like the buffeting/noise of the window down).

At lower speeds (<40mph), the opposite is true. I try to use P&G from work and I've tried it with the AC on 77/Auto and get about 55mpg. With the windows open (driver side full, passeneger 2-3 inches) and AC off I get 65+ mpg for the same road.

 

If you play with the windows up and down at highway speed, you can
more or less *hear* when the serious turbulence kicks in. Cracking
three of them an inch or two [yes, three, not all four -- the slight
imbalance gives better crossflow] seems to cause minimal additional
drag. I came the 1100+ miles back from Hybridfest this way, assuming
that any way I did it I'd just be a sweathog by the end of any given
leg regardless, and pulled into the driveway showing 68.9 avg on
the MFD.
.
_H*

 

impressive.

 

One Prius-specific detail I didn't see in this thread to keep in mind is that the A/C comforts the batteries as well as the occupants. Interior air cools the bats in summer and warms them in winter; they like the same temps that humans do. So, when you turn the AC off and put the windows down and it's hot outside, you're making the bats suffer, which apparently isn't as good for them as running the AC and getting them cooled.

///d@

 

<div class='quotetop'>QUOTE(speech path @ Jul 26 2006, 11:17 PM) [snapback]292965[/snapback]</div>

You're supposed to keep your key in one hand and your cell phone in the other, with your arms fully extended at all times!

I also read about this and questioned my dealer and others. Nobody said they ever had a problem, except that maybe if you have the cell phone between your key and the car, the signal could be blocked.

To be safe, I try to keep my phone and key at least a little separated, not right next to each other (different compartments in my handbag). Do this if you're a gal (or guy) and carry your keys/phone in your purse. Otherwise, carry them in opposite pockets.

 

If you are actually talking on your phone it will be transmitting and it could desensitize the SKS receiver in the fob if it were to close. How close would depend on the quality of the filters in the transmitter and the receiver, the power out of the phone, and several other factors. It also might make no difference whatever. If you are not talking or making a call the phone is most likely not transmitting and should have no effect on the SKS. If you had them in the same pocket and the cell phone antenna were positioned directly between the fob and the antenna in the car I suppose in theory it could absorb enough energy to reduce the range of the fob. Without knowing the frequency of the fob and the phone it's difficult to know much more than that.

I carry both (fob and Phone) in the same pocket and have never had a problem.

 

[deleted as superfluous] What dithermaster said. Anyone sweltering with the windows down at 95'F *may* be improving their fuel economy but they're probably reducing battery life and efficiency.

 

Ok, so what this is, um, boiling down to is that, if I am going to drive w/ driver's window open, which I do often, I have to "crack" TWO other windows as well to minimize bad effects? (And I don't usually drive w/ any open windows at highway speeds--too noisy to hear the MP3s! )

And this question CAN now be asked and relevant now that our recent LOOOONNNGGGG stretch of 100+ degree days has ended (for now)... :blink: