A/C Usage and Impact on FE

[bowtieguy]

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?