How does running the AC affect the fuel economy in these big diesels? I had an SUV before I traded it in on my truck and I used to dread burning the AC because I was already getting 14mpg and then it would bring it down to about 12mpg or so.

 

I too noticed that problem with my previous gas powered trucks. On my CTD, I let the A/C run most of the time in the summer. I really don't think it makes a difference on these trucks.

 

Last weekend I put on about 450 highway miles. 225 with the a/c on, and 225 with it off. With it on I averaged about 19mpg. With it off I got about 20, but it was about 10* cooler. I thought the mileage would drop a little bit, but I guess the truck didn't even know I had it on

 

You will see a bigger difference in stop and go traffic than in highway driving. It takes a lot of power to drive the engine fan in stop and go with the AC on. On the highway, you will probably get better mileage with the AC on than the window all open because of the drag.

 

On my 06, it takes about 1 mpg off my average around town (down to 16 from 17). I cant tell you on the highway since my highway mileage is consistantly less then my city mileage. The CTD loves to run 70, but with my 4x4 G56 dually, all I can ever get is around 15 highway.

Nav

 

Mythbusters did a show on this and found no difference (negligible). However that was on a gasser. I agree w/ JKitterman about city/hwy. I do see a difference when driving in town as opposed to on the hwy. But in AZ I will gladly trade 1mpg for sweating my butt off.

 

I believe the Mythbusters comparison was between AC and "windows open".

From a physics standpoint, of course running the AC has an effect. It adds load to the engine, and that extra load consumes more fuel.

However, from a practical standpoint, the effect seems quite small. I agree with the other responses that it's more severe in stop-and-go driving, likely because the engine spends more time idling and the ECM has to kick up the fuel flow to keep the RPM's at 750. At sustained highway speeds I haven't seen any change at all, probably because the extra load from the AC compressor is such a small percentage of the overall load on the engine when pushing the truck at 70 MPH. If it added 5% load, there are lots of other factors (winds, road surface, etc.) that could vary the MPG by 5% or more.

Bottom line: If you need the AC, use it.

 

[QUOTE=WACTD]I believe the Mythbusters comparison was between AC and "windows open".

I guess I assumed the windows would be open if no AC. When its hot and I decide not to use the a/c I typically roll the windows down...but thats just me, call me crazy.

 

Coming back from Utah last weekend with CC on 80, the OH said 20.2 mpg and when I turned on the AC and ran a few miles it showed 19.5 to 19.7 depending on hills.

 

The A/C load relative to engine output on these trucks is so small as to be considered negligible. Any difference you see on your overhead or at the pump could have just as easily been caused by 2 more stop and go cycles on a tank, a hard pull up a hill, 5 minutes more idling, blowing some black smoke in that ricer kid's face at the stoplight, etc. Does it cause less MPG? Sure, some, but we're probably talking a tenth or two of a gallon tops. It isn't like a 3 cylinder Geo Metro cranking on an AC system. A Cummins hardly knows it's there!

 

The increased aerodynamic drag from driving unairconditioned with windows down will create a slight decrease in fuel economy. It is about equal to the slight decrease driving with windows up and a/c running.

 

I can't tell a enough difference with either of mine to worry about. Driving habits, terrain and wind will have a MUCH bigger effect. Craig

 

I like to run my A/C at full blast, with the heat on, and all the windows down. I've been called an ecoterrorist by people at work