Windows: Up vs. Down

I've never tried, but I'd assume that at those speeds, having the windows down will create a lot of drag despite the aerodynamic shape of the body. When you have the windows down on the highway, you feel those gusts of air going into the cabin, which obviously will drag the car (kinda like a parachute). As to how much mpg difference, I don't think it would be too dramatic.

 

The weather is too unpredictable to come up with a constant for MPG loss due to wind resistance, or drag created by currents entering the cabin but over all from my own personal experience, the effect is multiplied when traveling above 50mph or so. The wind direction is also a factor, cross winds and head winds, are the devil for high mpg. Tail winds, are your best friend.

At or below 50 when I have my windows down I don't notice any extra drag no matter how bad the wind gusts. At 55-60-65 etc one gust, and you can watch the speedometer decrease by 1-2 mph easily causing you to have to press the accelerator pedal harder to regain that lost inertia.

After watching this keep happening to me on the highway, my usual instant mpg reading says 58, by the time I'm off the highway I'm on the mid 56.xx range. In reality, that duration of the trip most likely cost me 1-2 mpg in real calculations to enjoy the windows down on a windy day.

Tail winds can boost your mpg just as easily as cross winds and head winds can make it decrease windows, up or down.

As mentioned, none of this is a science just my observations.

 

All else being equal, you'll probably come out a little ahead with the windows being up. Not to mention, you'll be more comfortable most likely and still be able to hear when you get where you are going.

In my Maxima several years ago, it was an equal run for the most part on I-95 headed to FL. I ended up with roughly 30 MPG regardless of which method I used, so I chose to ride with the A/C turned on.

Remember, once the car cools down the MPG loss due to A/C is very minimal.

 

I haven't driven highway speeds with the windows down in a while. I remember driving 75 mph in Summertime Southern CA in my '95 Dodge Neon all 4 non-powered windows down in 110* weather bc the a/c was broken. It was chaotic now that I compare it to my Prius w/ windows up. Couldn't hear anything, couldn't talk with passengers, and any loose paper was flapping around inside the car.

 

In my previous Camry Hybrid, when having windows rolled up and running AC, I got ~ 3mpg less than with the right rear window opened ~ 1/2 inch.
On my commute (42+ mil on fwy) I always drive with right rear window ~ 1/2 inch down, except when raining. Here in Souther California it's never hot in the mornings (I start my commute at ~6:15), but evenings can be and that's when I use A/C.
When I have passengers in the car I usually roll all windows up, and almost always notice a (slight) decrease in mpg.

Hasn't been hot here since I have the Prius but I will experiment when hotter weather comes around

 

1/2 inch in the rear windows isn't gonna let much air or drag in. So any mpg hit if any would be very minimal.

 

You don't want to crack open the rear windows without at least doing the fronts or your ears will get shot.

 

I know what you're talking about, however, I do not get that effect, neither with 1 nor with both rear windows cracked

 

This was settled a long time ago... especially for aero sensitive cars like Prius. Windows down at Interstate speeds will kill MPG, conversation and what you barely hear from the sound system. Under 35 MPH, windows down not a big deal.

Driving a car at speed with windows down is tough on the ears. You can crack the back windows down just a bit in some cars and travel at highway speeds without having your eardrums sucked out of your head. I prefer the quiet of windows up travel... unless you're in a convertible or targa. That's totally different!

 

Drag doesn't necessarily just come from air entering the car; opening the windows will also disrupt the flow of air around the smooth lines of the car, causing more drag on the outside. Without having a wind tunnel, I can't really quantify that, however.

 

I 100% absolutely positively agree with you, my good sir. :cheer2::rockon::juggle::tea::whoo:eace::usa: :thumb:

 

Well, it isn't really a "debate". It tells us very clearly in the owners manual that you should drive with the windows up and the A/C on for best fuel economy. In the city not so much.

As example, when I travel a certain route in B.C. there is a very steep and long descent and I have found the best way to go down it is with the windows open for extra drag. It causes the top speed with no braking to be in the 110 km/hr area rather than -scary fast-. But if it's cold or raining I have to use "B" and the brakes.

BTW, this isn't Prius specific. Other vehicles I've owned perform the same, though the Prius -is- more aerodynamic with less drive line drag and does "take off" down the hill more than they did.

 

This is not a clear cut debate.

• In most cases when temperatures are mild and speed is high it is best to use AC.
• When speeds are low I would argue that windows down is better.
• When temperatures are very hot (100+degrees) it is best to drive windows down for the first few minutes until the cabin cools closer to ambient temps then switch to AC and roll up the windows.
• An alternative method is to drive with the windows down for the first few minutes then roll up the windows but leave the driver's side window cracked approx. 4" and the passenger side rear window cracked at 2"-4" as this will create airflow, reduce drag and power consumption. Or simply use the vent function with the AC turned off.

I will do some AC testing on the GenIII this summer when temps kick up but for now here is what I found with the GenII.


http://priuschat.com/forums/gen-ii-prius-fuel-economy/97814-how-reduce-mpg-loss-because-c-use.html


Methods for controlling heat buildup in your car:

1. Park in a shaded area. Under a tree may help but a better tactic is to park on the eastern side of a large building. As the sun moves to the west in the later portion of the day, the building will block the sun from hitting your car. With your windows cracked, the heat from the earlier part of the day will dissipate faster than if you parked your car in the full sun the entire day.
2. Tint your windows
3. Use a high quality windshield sun shade. Priuschat shop has a very nice one!
4. If you cannot park on the east side of a building try parking your car facing the west and use a quality windshield sun shade. This will reduce the number of windows directly facing the sun and thus not allowing as much of the suns radiation to enter the car.
5. Drive the car for the first few minutes with all the windows down to circulate the hot air and direct it outside.
6. Buy a Prius with the solar roof option

Best Practices for A/C usage and maintenance:

1. Use the A/C sparingly
2. Use a setting that is high enough for maximum efficiency but makes you comfortable. 78 degrees seems to be sufficient for most drivers
3. Ensure your A/C system is charged to spec levels and operating efficiently. i.e. condenser is clear of obstructions, bugs, etc.

 

Coming down the Coquihalla into Hope? Yeah, that's quite the hill. As for 'scary fast', you should try it on a bicycle. I wish my speedometer went past 99.5.

As for the window issue, I find it too noisy on the highway to have them down. They're usually down in Summer around town, but it's rarely what I would call 'too hot' around here, anyway.

 

My favorite, is windows down AND the AC running. :rockon:

 

KMH, not MPH. My only regret is not seeing the look on the face of the trucker I passed. I was too busy looking where I was going.