It was obviously naive for me to think the A/C or auto mode didnt use more fuel than it being completely off but a 4-5 MPG difference! I am experimenting and that is what I am coming up with. I can now do 27 mpg on the highway, I was amazed. Anyone else compare MPG's with AC on and off?

 

Contrary to what government bureaucrats state on the fueleconomy.gov website and others, turning on your A/C at highway speeds versus rolling down your windows does NOT save fuel. That 4-5 mph figure seems a little high, but I believe it. A friend of mine tested his Kia Sportage V6 at 55, 60, 65 and 70 mph. The fuel consumption at 70 mph with the windows down A/C off was lower than at 55 mph with the A/C on.

I bet in my Jag, the difference between A/C on and A/C off is about 3-4 mpg, close to your figure.

 

During the winter months I routinely go to the manual mode and turn off the a/c. Seems to improve mileage by about 2-3 mpg.

Mike

 

Should we suggest this to the mythbusters?

 

Love that show...I always keep my car on "auto" 68 degrees, looks like I will be driving with the windows down more often....

Another thing.........When the climate control is off, I have no knowledge of the outside temp which is extremely aggravating.

 

I think that mpg with the A/C off vs. on is subject to numerous variables including outside temperature, elevation, road conditions, general terrain, highway vs. city driving, personal driving habits and style, etc.

When my wife takes our S-Type back and forth to visit her parents in Florida, she usually averages more than 33 mpg using cruise control set between 72 and 74 mph, and she always sets the HVAC system to hold the cabin temperature at about 70 degrees (heat in winter, A/C in summer). Mpg on those trips is slightly better in winter when the outside air is colder and therefore mixes and burns more efficiently in the cylinders. I never expected this car to be able to achieve that level of gas mileage. It was an unexpected benefit that we're glad to have....

 

Now, if the outside temp is 65, my "auto" is set to 68, and the cabin is flirting between 65-70. Does the the AC turn off completely if it is warming the car back up to 68 degrees?

 

Hang on. On a previous but similar V6 car to the 3.0, I tried to measure the effect of CC because I had been told it was 2mpg - well, I could not measure it even as 0.2mpg.

I haven't yet measured the jag (mine is an STR) but will do in due course.

I can well believe that PEAK is (say) 2mpg or more but on the average???

To be 2mpg (never mind 4-5mpg) where the heck is all the power supposed to be going?

 

No idea man, but its true (at least for me). I have NEVER came close to 27mpg on the highway, not even close. More like 23. Granted I have CC on ALL the time. For Sh*ts and giggles i decided to run it with no CC the past 3 days to and from work on the highway and am consistenly getting the same results. Needless to say, I was shocked, and thats why I had to create this thread to find out if I had in fact finally lost my mind.

 

I have not been as thorough as some of you, but I find very little difference AC on and off. But temperatures are cooler here, 20 degrees celsius or 68 degrees fahrenheit is a good day.

 

I have riden mine on long distances (500 miles0 and found mieage goes down about 4mpg,s useing a/c without I get around 29 or 30 mpg

 

When Jeremy Clarkson did the Audi A8 run from London - Edinburgh AND back on one tank of fuel ( over 800 miles ) he clearly stated that using the A/C will result in around 6mpg less.
Driving with the window down will cost you approx 5% in your fuel economy..... heated rear windows and heated seats ALL eat into your fuel economy too.......Please see the attached links

http://www.youtube.com/watch?v=GIHdpmTIrFw part 1

http://www.youtube.com/watch?v=_JO-GhYkwrM part 2


I haven't thoroughly checked that theory out but just a brief test a couple of times seemed to get me more mpg for sure!

 

The AC compressor is rarely engaged when heating the cabin so I fail to see how it could drag down mileage by anything measurable.

 

Can we be sure that JC never tells porkys in the interests of cinematographic drama?