Mythbusters (the TV show) proved it's MUCH more than .25 mpg, it's more like 25% worse fuel mileage. Episode 22 (Nov. 10, 2004) of Mythbusters proved that running the A/C was HORRIBLE on gas mileage period using identical vehicles with identical amounts of gas. If all you lost was ¼ mpg every vendor would have it on. It's not on in my MINI or my Subaru unless I turn it on or it's on full defrost but when I leave full defrost it's off.

In fact, the manual says Nissan leaves it on at any temp above 36°F (37 in 04) until you turn the blower off (in 04 it stays on until you turn the X off). It's a crappy design IMHO and needs to be revisited. I've considered a simple cut off switch to the A/C fuse but haven't been quite gutsy enough to add it just yet.