I do 100 miles a day on the highway doing 70+MPH.My windows are down even in the winter.
When it gets hot I close the windows, turn on the A/C.
My MPG drops when the A/C is on, windows up.
I thought it was better to run the A/C on the highway, than windows down

It does not make a big change in MPG with the A/C on, but I do notice a drop.

The diff. between windows up/windows down, with A/C off is nothing that I can see.

 

The Mythbusters did an experiment a while back that basically showed that identical vehicles over a long bunch of miles driven at constant speed on a track didn't have a significant advantage over windows or A/C.

Seat of pants feeling is a little different. I too have a slight drop, and while I think that highway speeds with cruise on there may not be much difference, I think that running from stop is where you hit your increased consumption since your load is higher.

But I don't claim to know anything.

 

Umm, I saw that episode too, and I remember that they did prove a difference. The SUV [] that they used to test went like 30 miles or more further on a tank when running on cruise control with no AC versus the identical one that ran with AC.

The truth about AC is it's another thing driven by belt that must be turned along with the rest of your engine's accessories. So, AC off shows less resistance along the circuit of belts than when your AC motor is engaged; the pulley stops spinning free and begins having more resistance against which to turn when you engage the AC.

So, by a small but noticeable degree, the car must make more power to proceed at a uniform speed with the resistance on the AC pulley than it would need to make if the AC were off. Less power needed means less fuel delivered, means longer cruising. It works out to two MPG or so.

ALSO, it also is easy to see how MPG will suffer even more in a car with the AC on when it has to accelerate, and not just cruise. Acceleration requires much more power to be made to turn the pulleys than does cruising. Therefore, any baseline difference between AC and no AC while cruising is amplified when you are having to accelerate. The MPG differential gets larger around town.

Any thoughts?

 

Along those lines, I've heard that running a heavy electrical load will drop your mileage, albeit at a lower rate than running the A/C. That doesn't seem to make much sense to me as the alternator is always running - it's not clutched or anything, and I don't see how running a big amp or adding a brace of lights would increase the drag on the alternator. . .can someone enlighten me on THAT myth? Thanks. . .

 

A/C on or off. Lot- you need to do a miserable run with the windows down in the heat, and then a run with the A/C on and the outside temp the same. The engine is not as efficient in hot weather.

Cruise vs acceleration. If the acceleration is hard enough, the A/C compressor will disengage. Because of the amount of heat the refrigerant in the system can absorb, unless it is really hot, you will not notice any warming of the air for about 10 seconds. You can verify this by having your passenger turn off the A/C without telling you and seeing when you notice.

Alternator drag. Only when it is producing electricity. You have to be cranking out 50-60 Amps before you are using 1 HP in electricity.

 

So putting 17 driving lights and a 2,000 Watt amp on my Briggs and Stratton lawn mower ain't that good of an idea??

 

Bill, you'll definately kill your YPG ratio (yards per gallon).

 

Rather than add clarity to this windows down versus AC discussion, I’ll point out that the solution depends not only on the drag added by the AC but the increase in drag with the windows down. Although the MINI looks blunt, it as a relatively slippery drag coefficient of 0.35 (windows up). I don’t know how much that goes up with the windows down. The Mythbuster SUV probably has the same drag coefficient as a parachute with the windows up or down.

 

Trust me, it's worth it to use the A/C. A .35 Cd is not very good and windows down will absolutely kill it. My other car is a Honda Insight (.26 Cd) and this topic has been discussed over on their forums too. Up to a certain speed (say 35-40 mph) windows down will probably be alright as I've been able to get over 100 mpg with windows down at those speeds (in the Insight ). Above that, the Cd really takes control and A/C is really the best way to go.

 

Honestly, it's worth it to run the AC for me, too, every once in a while. It's simply too hot here in the summer. But, the test used one machine windows open [no AC] and one closed [AC] and the windows open one went significantly further. The COD does factor in, for sure. But all-out MPG wise, I'd say that the drag in our MINIs, too, would not be enough to overcome the driveline loss. I admit, though, that the AC sure is pleasant feeling .

 

I put about 1200 miles a week on my Mini, mostly highway. I think there is a small 1-2mpg difference between windows up, no AC and windows up, AC on. But there is a 2-3mpg difference between windows up and windows down. I'd rather have the AC on at 90mph anyway.


Has anyone here seen that science project where some kid put two big vertical "wings" on the back of the hatch itself and increased gas mileage by leaps and bounds?

 

Ok, that settles it then. When it's hot I'll turn on the A/C, when it's not I'll motor with the windows down. Either way I'm getting 35-38 MPG, so I'm not really worried about MPG.