Better MPG with windows up?

Over the last 9k miles I have tested this "windows vs. A/C" theory.

I personally have found that if I have the driver's window down and the back passenger driver's side window cracked around 2-3 inches I am able to maintain my normal average of 51-54 MPG (the same as when I drive with all windows up).

If I put all four windows down I do notice a fairly significant drop in MPG to 48-50 MPG...and I can "feel" the drag created, especially when at freeway speeds (I drive around 50% freeway - around 700 miles/wk).

If I use the A/C on LO then I normally average 48-49 MPG...very close to the "all windows open" test.

So for me it boils down to outside temperature...if it is too hot to just have my driver's side window down, then I normally switch to A/C rather then lower the passenger side windows.

 

Interesting. I've been playing around with this as well. Using both driver and passenger window down and I can feel the drag in the car. With only one window down the drag is a lot less but not much air circulates.

I guess I should try using the AC at ~78 for a couple of days and see how it goes.

Most of my commute is 35-40mph.

 

<div class='quotetop'>QUOTE(SomervillePrius @ Jun 23 2006, 11:03 AM) [snapback]275822[/snapback]</div>

I've been wondering about this. I hear using A/C is better than putting the windows down, but in most cars you can feel the engine work harder the second you put on the A/C, so it seems hard to believe.

But it depends what they mean by windows down. I tend to have the driver's window and the back right window down about 2" each (4" for hotter weather), and get good circulation throughout. I think this would still keep most of the aerodynamic benefits, since two windows are still closed and the other two windows are 60% closed. My route is too variable to be able to test this with any accuracy (unless somebody gets the CANView working for the 2006).

 

I agree that running the AC costs power but the Prius AC seems fairly efficent so it comes down to drag vs. ac power.

It's hard to tell how the Aerodunamics gets affected even with the windows not fully down. With them half way down it feels like less drag but I think I can still feel the drag.

Here's my thinking (based on no data)

<25 mph - Windows down makes no or little different as aerodynamics effect is low. I think at this speed the running the AC would cost you mpg
26-40 - this is the hard range. Aerodynamics starts to play in, not clear how much. This is my commuting speed and I can't seem to find the best solution. It's likley there is none as they are about equal.
>40+ - AC is def. more effective then havning windows down

Interestingly the same goes for the cruise control. Up to about 45 mph I get better gas milage with pulse and glide. At around 45 mph it's to hard to use well and uses to much battery so the cruise control is better. I tend to use the cruise control on flat sections and down-hill then either 'blip' the speed down going up the hill (so that the engine don't rev to hard) or release the cruise control and try to stick to a mpg number (normally around 30 mpg) and let the car slow down as it wants to.

/Robert

 

The basic thing to remember is that drag has more of an effect at higher speeds. In the equation for drag, the velocity contribution is squared, whereas the drag coefficient (altered by windows up vs. windows down) is not squared. What I tend to do is at lower speeds, I'll have the windows down (assuming the temperature is bearable, we finally hit 100 yesterday, and even in the dry weather, that can get uncomfortable), and I'll switch to A/C when I'm going faster. My cutoff is usually between 45 and 55, depending on how many smog spewers are in front of me during that part of my commute.

<div class='quotetop'>QUOTE(SomervillePrius @ Jun 23 2006, 09:48 AM) [snapback]275850[/snapback]</div>

Here at least, with all of the hills, I don't do as good with cruise control, just because I can see what's coming up ahead and work with it, whereas the cruise control only knows what it's doing right now. I can accelerate a bit more before going up a hill, and let the momentum take me up, and then do the reverse going down a hill.

 

I was driving about 40 minutes a few weeks ago, on the highway at cruise set at 60 mph. My mileage spiked so rapidly until I was over 60 mpg. I was so excited but haven't yet been able to duplicate this reading, no matter how efficiently I try to drive the car. Of course, I've spent much time trying to figure it out and it's occurred to me that this phenomenon was probably due to the wind being on my back. My windows are usually always open as they were on this day too - I like warmth, lots of it!

 

It could be that your car is going through a break in period. I notice you have only 3100 miles so if your experience is like mine mileage will continue to improve through around 10,000 miles or so. I also drive in and around Pittsburgh so under similar conditions I easily get over 50 mpg in warm weather. Today the MFD read 53.4 mpg when I arrived at work in Oakland.

 

Some of the windows up/down thing for me has to do with the wind noise. The stereo isn't nearly loud enough to drown out the buffeting. (No, I'm not deaf...yet.) Around town, or on a short trip, I don't use the ac even if it is hot. On a long trip, air conditioning adds so much to comfort that I consider it a safety feature.

 

<div class='quotetop'>QUOTE(TonyPSchaefer @ Jun 23 2006, 12:13 PM) [snapback]275865[/snapback]</div>

We're both nerds, and that's pretty close. All we need here is an understanding of statistical variance and root cause.

So this is taking the car's warm-up period into account? My first 5 minute block is always in the 30's in this weather, then it jumps to about 60 after that. It doesn't matter what I do with the windows or A/C. Of course, I know you know that, but maybe you've been temporarily blinded by paying too much attention to one detail? But since you're looking at the instantaneous reading, the engine warm-up can't be accounting for all of the change.

(BTW, I got 57.8 mpg on the last tank, all normal commuting - this warm weather is great! B) )