weird gas milage ...

[Maverick]

okay, heres a new one for me at least...

I get about a 10% difference in mpgs between a/c on and a/c off.. sounds about normal right?? Wrong, i get better milage with the A/C ON!!

at first i thought it was a fluke, so i started experimenting, same gas station, same grade (regular) power switch on..alternating with the a/c on and off, by the tank full, about four cycles now..my milage is definatley allot better witht he a/c on..
its just plain strange, what gives??

[Ldub]

One possibility : The increase in mileage may be due to less wind resistance (a/c on, windows closed...a/c off, windows open?)
Driving at speed with an open window creates lots of drag.

Just one of my crackpot theories, Ldub

[Tone]

Myth Busters just did a show on that and that is correct for most modern vehicles - due to drag with the windows open, better mileage is achieved with the AC on. In town driving though is different, with the compressor off there is less drag on the engine and aerodynamics have little effect.

[Maverick]

i thought about that, but the thing is, its been cool, i have ventshades,so generally i leave the windows cracked with the a/c on or off and just adjust the temp to suit me ( i like the fresh air) .. Soooo , while buffeting may have had some impact, i really dont think it was enough to explain the differences. no real major changes in my driving habit/configuration this time of year. in fact, ive had the windows closed and ac off this past tank, as its been cold , and again, worse milage than when its on.. i drive over 600 miles a week just commuting to work and back, so i can test this again in no time..but im due for a mobil one change again in a few days, and an k&n "wash" so ill let the truck settle in again after that before i report back on this..

[Tone]

Cold climates / temps can also lower mileage due the colder, denser air that the computer compensates for by adding more fuel to the mix to keep from leaning out too much.