A/C Usage and Impact on FE
 

now that summer is here, and 3 digit temps(heat index) with it, it is suprising how much impact my car's a/c has on FE.

let me first break down a/c on vs a/c off on the scangauge. this is at standing idle only since variables are eliminated...

IGN on: 14, off: 12
LOD on: 20, off: 15
GPH on: .4, off: .2

i cannot comprehend that gal/hr is doubled at idle w/ the air on. obviously w/ throttle and driving situations the % is no where near that but...

according to SGII, there can be as much as 4mpg difference in the same trip, EXACT conditions. i ran the air on the way to work the past 2 days yielding ~36 and 37 mpg. typically i'll get ~40-42 air off.

i am accounting for a bit a variables, so i dare not claim a 6mpg difference. remember, everything has been nearly exact and the same--route, weather, breeze(none), etc. typically, this time of year, my only variables are traffic lights. i time them very well, and usually make them all but 1 or 2.

so, at 2mpg, the loss is ~5%, and 4mpg it is ~10%. maybe closer to 4 and 9% respectively. still, that is significant. am i figuring this correctly?

At idle it only needs to make enough power to overcome engine friction, which isn't much at idle RPM and closed throttle. I can see A/C being as much as that.

MPG impact is directly related to engine power.

Larger engines don't get affected as much by ac. I would venture to say that the MPG impact is about as noticable as the performance hit. That's what my experience has been anyways.

Are there any cars that kick off the A/C when they need more power? I used to do it manually when climbing the steep hills in Providence in my 87 Sentra.

i think you're spot on there. i don't remeber my V6 olds losing that much. besides if the % was the same, obviously the loss in larger engines and lesser FE would not be as noticable.

AFAIK, most modern cars turn off A/C at WOT.

I think larger engines don't suffer as much... I tested this a year or 2 ago. Drove my truck on its daily drive, with the A/C on max for a week, then drove it with the a/c off and windows up for a week and noticed no difference in my mileage.

-Jay

Chryslers turned the a/c off under heavy loads in the 80's. I hated it. Hit a good sized hill and the a/c would shut off. I'd hate to have to drive it up a mountain in the summer...

I hadn't considered that. Pretty ironic given where I live!

Yea, but you are in Florida too.

I have actually only needed to use mine a couple of times this year and mine makes no difference on the highway where 95% of my driving is.