Does A/C hurt fuel mileage?

[CGauthier]

As the topic suggests, I got around to wondering if running the a/c hurts the fuel mileage on our trucks, In my mind i came up with no with the reasoning that with all the compression and torque the CTD makes i figure no way it makes any noticeable difference having to turn that one extra pulley, all though i have no idea how much drag the compressor brings to the table. Any one got any solid evidence that could help convict the defendant of robbing millions of innocent people of their ever so important fuel efficiency in a day and age when you wish your vehicle could run on gold instead?



Carl G.

 

[JLEONARD]

If it needs horsepower to run it will affect mileage. Plain and simple. And A/C requires hp.

Now if the windows are open because now the A/C is off, then that's a different issue to consider.

I saw a quick blurb on the local news last year I think it was that said with the newer cars having the windows down created more drag hence hurt mileage more than running the A/C.

Not sure how those aerodynamics would affect our bricks.

Good topic for the Mythbusters.

Jay

 

[AKimmel]

Mythbuster's have done it. I want to say they drove 30 more miles with the window's down and the A/C off.

 

[lcaad1249]

I think mythbusters proved the exact opposite. It was two similar ford explorers and the A/C windows up was best for mileage.

 

[DonTX]

Checking the web, it seems to depend on the speed and the vehicle. Here is an interesting link on the subject: http://www.bankrate.com/brm/news/auto/20050804a1.asp

 

[radcam]

If you are at Hiway speeds then it is better to run the ac as opposed to having the windows down. If you are in town at lower speeds, stop and go traffic then it is better to keep the windows down and ac off. This is according to a study that was done in the 80's. Of course if you live where it gets real hot (we are supposed to hit 106 today) you might be better off just running the ac all the time and trade a little comfort for fuel economy.

 

[JLEONARD]

just running the ac all the time and trade a little comfort for fuel economy

Or better yet just drive thru town over 40 mph. All for the sake of being "green" of course. Sounds like the eco"logical" thing to do. :-laf

Jay

 

[PKitzman]

I don't remember the exact figure, but on my truck the extra load of the ac unit is about 4% according to the display on my Edge unit. If I leave the ac on in 100+ degree ambient temps my EGT will not drop below 400 at idle in park. If I shut the ac off it drops the load on the engine enough to drop the EGT to around 350 - 360.



I would bet that running the AC will drop your milage 0. 2 - 0. 5 mpg.

 

[RWWinslow]

A good rule of thumb to remember is:



From 0 mph to 55 mph, fuel economy is affected by the rolling resistance of the tires, driveline efficiency and parasitic loads on the engine (i. e. A/C).



From 55 mph to 65 mph, fuel economy is affected by both driveline efficiency and air resistance.



From 65 mph up, fuel economy is affected solely by air resistance (it is huge).



In other words, if your truck gets 22 mpg at 55 mph this mileage is due to driveline efficiency. The same truck will then get lower mileage at 65 mph and 75 mph due to the additive effects of wind resistance.