Will rolling down windows save fuel or not?

My experience has been that in local driving I roll the windows down and on the highway I use the AC - Unless it is super hot then the AC is always on.

At low speeds the AC puts more of a demand on the engine and will cut into mpg proportionately more than at high speed. This is slightly less true for the electric compressor in the new Prius. Also at low speed wind resistance is not a big factor.

At high speed the car becomes more aerodynamic with the windows closed and you save some that way which compensates for the power used to run the AC. Now there are some that claim that the battery needs to be cool and you will have more battery power with the AC on. I have not seen this in real life - but as they say YMMV.

 

Bottom line- best mileage is achieved without AC at any speed. At higher speeds the windows are simply rolled down a few inches instead of all the way down.

 

I Agree 100% Ray. It was 95F here yesterday and I enjoyed the summer breeze blowing through my hair (whats left of it). Those who are even more fanatical will drive with the windows closed and wear an ice vest. Now that is commitment.

I will stick with a breeze.

Dan

 

I agree with kirbinster; local driving is when I roll the windows down. When the speed gets above 50, I start thinking about rolling the windows up and turning on the a/c. Open windows make the car less aerodynamic but it's not noticeable, at least to my mpg, at local speeds. I've also rolled up the windows at slow speeds when it's very windy. Would windows down with very windy conditions and local driving be similar windows down at high speeds?

Bankrate fails to qualify the statement "Air conditioning can drag down your car's fuel economy by 10 percent to 20 percent. " They don't say at which settings their range applies to, which vehicles, how the vehicle is driven, etc. They also don't counter that statement with how much having the windows rolled down at high speeds subtracts from mileage so their information is incomplete and somewhat superficial.

 

If you read the article, the windows down reduces economy at speeds above 45mph by 10% and that equates to the AC drag of 10%; the article notes that in excess of 55, aerodynamic drag can exceed 20% so AC makes more sense:

"But as your speed increases to 45 mph, or highway speeds," says Jason Toews, co-founder of GasBuddy.com, "wind drag becomes an issue. Driving with the windows down increases the drag on your vehicle, resulting in decreased fuel economy by up to 10 percent. Drive at speeds over 55 mph with windows down and you'll decrease fuel economy by up to 20 percent or greater."

A little different than the initial Aerometer's statement that introduced this thread:

"It said "go easy on the air-conditioning. Roll down your car's windows and let in the summer breeze. Using the gas-hogging air conditioning as sparingly as possible will give your car's fuel economy a real boost. Air conditioning can drag down your car's fuel economy by 10 percent to 20 percent. "

I suspect that our Prius's AC system is a bit more efficient than the SUV's systems quoted in the same article...

Here again is that trade off between comfort (and for some with breathing problems) and health, and the desire to squeak 0.05mpg more.

 

Ooops, that should have referenced 'Areometer' vice 'Aerometer'...

sorry

 

Remember that much of the cooling effect from a stiff breeze on your body is from evaporation, which is not able to help cool the battery (the battery has no sweat glands). Also, the intake vent for the nice expensive traction battery is inside the passenger compartment. Clearly Toyota expects that outside air will not always be cool enough to suit the nice expensive traction battery. Another factor is that air coming in the windows doesn't get filtered; some fraction of that dust will accumulate inside the battery compartment and further reduce cooling effectiveness.

 

I think Toyota chose to vent intake air from the inside because thats what made the most sense. An intake from the outside would require more ducting and some sort of filtration. Inside air is already protected from the elements.

In my opinion, the traction battery does not require air conditioning to keep it safe. The cooling system is quite adequate without requiring conditioned air. And if it was not adequate, the Hybrid and Battery ECU's are quite capable of preventing any damage by limiting the amount of current flowing to and from the battery.

If AC was required to keep the battery safe, without a doubt, Toyota Engineers would identified this as a problem during the Failure Modes and Effects Analysis of the HV system. A suitable resolution to the failure mode would not have been "we assume that the driver will use AC".

If you can't tell already, I don't use my AC.

Dan

 

That's no doubt true. Of course, limiting the current flowing in and out of the battery unavoidably makes the hybrid system less effective, which reduces fuel economy.

As you can probably tell I never drive with the windows open :_>

 

It seems that since both Richard and Dan are holding themselves out as examples of how to use the system and since they both say that their way is better for fuel economy and since Dan's fuel economy is 50% better than Richard's and since my own experience and testing agrees with Dan's, I'm gonna have to go with Dan on this one.

I really respect Richard though and enjoy and learn from all of his posts.

 

On a short trip at low speeds, I roll down the windows. On a long trip at high speeds, I use the air conditioning. It's cooler, obviously, but also quieter. On our recent road trip, I came to consider air conditioning as a safety feature: less driver fatigue, and less passenger fatigue, which means less fights in the backseat. But I don't just crank it to max and leave it there, because I know there's a fuel cost, and a power loss. There are other techniques to help the efficiency. I tend to turn it off going up a big hill, and back on going down the hill. When first starting out, I roll down the windows to let the hot air out, and then turn on the air. A parking spot in the shade makes a huge difference, and I leave the windows down a bit to keep the interior from roasting.

 

hyo silver, unless you have a classic (like me) I don't think turning off the compressor when climbing a hill should make any difference. The Prius-II has an electric compressor, not a shaft driven one, so it does not add any drag to the engine when climbing a hill. I have done what you suggest and can feel it help the car climb a hill, and it really does not make much if any difference in the car's temperature.

 

I was once told that driving with the windows down creates a drag and lowers gas milage and that you are actually better off with the air conditioner on low and windows up.

 

Do you know about the little switch that you can change from outside air to recirculate. How about making a little scoop in the engine bay that can be opened or closed accordingly just like recirculate and outside air, that will push air through the vents in the car without any fans? I would kill to have that option on any of my cars!

 

This is true. However, the amount of drag below 45 or 50 mpg is generally thought to more efficient on gas mileage than the AC. Above 45 or 50, the AC is probably more efficient.

 

In response to another post, I shut off the AUTO feature and then pressed the fresh air intake. I definitely felt air coming in simply based upon the forward movement of the car. When I pressed the recirc button, air stopped coming in altogether.

So this already happens, or at least it does in my 05. Mind you, there wasn't much air moving, but enough to cause an active air change.

 

Well, I don't have a prius yet to know that. But I'm happy to know it. How much air comes out of the vent when on recirc and fans off? ex. compared to the first fan setting? Thanks

 

If the AUTO is off, and the recirc is on, zero air comes out of the vents. Actually, the first fan setting, if blown right on your face or chest, is not bad.

If the refresh is pressed, then a small amount of air comes out. Certainly not enough to be "refreshed" by it, but it will be different air.