MPG with Air Conditioning vs. Open Windows

Mythbusters and some other above average tests have been run. It appears that at high speeds, windows up AC on is about equal to windows down, so it may cost a little, but not much more to run AC. Around town, stop/go, speeds under 35 mph, windows down is better.

Hypermilers (the guys who routinely get 70 mpg) drive with the ac disconnected and the windows up. That also keeps people from asking you to drive them places and that saves more gas.

What is a 'hypermiler'? is it just a name for someone who does long journeys?

A hypermiler is a person that goes to the extremes some legal and some not to get the highest mpg's possible. Some of the things they do is rolling through stop signs and intersection to avoid having to start from a dead stop. tailgating to used the draft to name a couple. Most of what they do is not recommended and in some cases very dangerous and illegal just to try and save a little bit of gas. You can learn more with a simple search on the net.

The sweet spot probably varies from vehicle to vehicle.

In the Mythbusters episode, they used two SUV's and the sweet spot was around 45mph. Below that, better to drive with the windows down and the AC off. Above that, windows up with the AC on actually performs better.

However, as TPV45 says, running with the windows up and the AC off will surely give you better mileage. Just don't ask me to ride with you.

I'm answering my own post but, just to prove a point:

I drive very conservatively. Slow acceleration, high gear at lowest speed possible, drive at 60 to 80 km/hr, avoid sudden stops, etc.

My old clunker used to get around 200km per P1,000 (that's one thousand pesos, about US$22) worth of gas (before the prices started rising-good ol' days). When I stopped using the air conditioning in the morning and afternoons (going to and from work), my P1,000 got an additional 30 to 50 kms. I still used the air conditioning during Sundays when we go to church and to the mall but that was about it. 6 days a week with no air conditioning.

If I needed any proof that running with the AC off increases mileage, this is it.

yes.

There is certainly an mpg advantage from driving without the AC on. It consumes energy and that energy comes from the gas in addition to the energy required to move your rig. Regarding driving at higher speeds, if you are driving so fast that the drag from the windows down is significant, then it is possible that the drag even with the windows up will be high. The answer here is to not go so fast. If you drive in a calm manner, plan your journeys, allow for delays, and have a more relaxed outlook on daily tasks (set reasonable goals), then you may not need so much cooling down in the first place, will drive more economically and perhaps even live a bit longer!

Its just a shame that the majority of people have had to wait for increasing fuel prices to spur them into such actions.

You try commuting 95 miles in 95 to 105 degree heat, and tell me you would drive slower than 75 on the highway, or would consistently not use your a/c!!!

It would depend on the vehicle type and operating conditions.

Please share these with us.

Vehicles are: small pickup truck, Large Minivan, small 2 door coupe.

Conditions are: Lots of hills, hot and dry.

Max Speed: 55 MPH

Still not much to go on.

Are any of these vehicles designed for a low CD (drag coefficient)? An example of one that is would be the Toyota Prius, Alpha Romeo Spider, and many newer cars. The CD will be adversely affected by open windows and moreso at high cruise speed. You mentioned 55 mph so the impact is less than it would be at 75 mph for example.

Minivans do not have favorable CD's so opening the window won't make things much worse.

Pickups have poor aerodynamics generally, but again you won't tell us much about the truck so it is hard to tell with certainty.

An old rule of thumb was that the penalty for A/C was in the range of 10 percent due to drag on the crankshaft from the compressor motor. This may be less depending on the equipment used.

You did disclose that the area is hot and dry. Matters could be worse of course. For example hot and humid.

Any loads including the A/C compressor or towing a trailer will rob gas (I am assuming these vehicles are not deisel) mileage on the upgrade so turn the A/C off when climbing.

Coast down hill in gear. If the vehicles have a standard shift overdrive gear use it.

I read you all learned ones with interest.

I guess the question was how much saving is there with rooled up windows and windows down and A/C on. I read some where and then I heard that the saving in absolute numbers was Approximately 6%. That mean if you rolled up the windows and A/C off, you can get a mileage about 6% higher (I am sorry for over simplification guys and ladies).

By the way, I ALWAYS, I mean always drive with windows down and A/C on depending upon the time of the year and specially in Houtson, TX. Without sounding like a snob, I wold say that I do not care how much the gas cost as I have bought the car for my comfort and not to be slave of the weather or the damn oil companies. Yes, it hurts to fill up my Ford Focus SE for 45-48 dollars but again it is for my own comfort and driving habits. Also I do not like the wind blowing thru my face and shoo- shoo sound of passing cars and trucks piercing my ears. Shoo may be denbated but I do not know how else to describe the sound of passing cars and trucks at 60 -70 MPH.

Regards;

Nadeem Butt

06202008

It all depends on the power of the engine. My wife has a mini van with a 3.8 L that gets 21 mpg on the hwy with the air on and fully loaded with passangers. The same vehicle and loading gets 28 mpg with the air off. The lower the power output of the engine the greater the increase in mpg for the most part.

How about windows just crack open to allow some fresh air and AC off ?

But to reduce drag , one should also look at Side mirrirs and roof carrier creating drags.?

OR

How about cars designed to get in fresh air thru a small aperture just between top aerofoil roof and litlle gap created over the windows at back and front ? That is air moves out staright thru the bottom of roof and unless passengers require the air it just moves out. So In and Out is straight. Now If we are looking for some comfort open a bit of roof and get fresh air.

With Oil price killing us , we need to think many probabale options to save money.