Quote Originally Posted by Wubbalubbadubdub View Post
It does this because if you decelerate in neutral the engine needs to keep running and getting fuel to maintain idle. When you decelerate in gear the vacuum and throttle position sensor at zero tell the computer you are decelerating so it turns on Deceleration Fuel Cut Off. You don't need fuel to maintain the idle because the vehicle speed will turn the engine.

I've tuned a lot of GM lsx vehicles and you can't get as aggressive as the Mirage's factory DFCO settings because the big v8 will slow the car down so much it's a problem. In my Mirage? I leave it in gear and let off as much as possible. Doing so will maximize fuel savings.

That being said....I had DFCO turned on aggressively in my 02 camaro with ls1 and 4l60 automatic. Used to average 23-4mpg in an auto!! That car ran 12.5@112 with nothing more than long tube headers and tuning for 93. Dyno'd 340 at the wheel. If it had been a 6 speed I could see getting 25+ no sweat. A lot of guys on stock engine ls1 t56 cars were knocking on 30mpg on the highway without things like cylinder deactivation simply because 6th gear was so tall and the ls1 had more than enough torque to propel the car at highways speeds well below 2k rpm.

People I tuned for didn't like aggressive DFCO usually so I would either disable it or keep it as soft as factory. I can feel it in the Mirage but it's not like the engine braking feel I used to get in my camaro. My camaro would slow down so fast without brakes I had to watch people behind me to make sure they were paying attention.

My old camaro running the 1/8th mile https://www.youtube.com/watch?v=SmZHrLpBY28
Good post, welcome to the forum. Hilarious name, btw.