dfco
If you have a manual trans., I would suggest to leave it on(I'm not sure how this works on an auto). What happens is when you leave the car in gear and decelerating while above 1500rpm or so, fuel is totally cut off. On the gto, you can tell when this works because the instant fuel mileage will go to 99mpg. If you turn this off, then the mpg would be just like idle mpg while moving. The only time you would turn it off is when you are tuning. This way, the lower rpm cells will not show a high lean condition. Hope this makes sense.
Works relatively the same on autos. If you're in (D), it will kick in when you're going fast enough and take your foot off the gas for a few seconds. You'll feel it more in D, and even more in 2, and the most in 1. The car also goes into an open-loop state during dfco, so stft's go to 0 and ltft's may go positive or negative depending on how your car likes your tune.
I takes a second or two for it to kick in. On my car I can clearly tell when it kicks in. There is a noticeable increase in engine braking and the exhaust note changes noticeably. I would recommend leaving it on.
