Well said Speed Racer, you sound like a mechanical engineer and I appreciate the explanation. I guess that is why my JSW TDI w/ dsg feels so different than my other car which is an automatic Nissan Altima. The Altima feels like it rolls much easier at slow speeds w/o touching the gas pedal, barely noticeable engine braking compared to the VW.
In D the Altima starts rolling as soon as you let off the brake but the VW just wants to sit there until you hit the throttle.
I suppose the VW's turbo does not really engage until you press the accelerator which takes some getting used to when switching from 1 car to the other, though at speed the response in the VW seems instant whereas it barely wants to move when in D w/ no foot on the gas on level or uphill grade. I guess that is just the difference in the engineering of the two.
I'm not a mechanical engineer or anything. Just a copywriter with a life-long interest in all things automotive and a thick streak of car geek
Turbospool1 is right, it probably makes a negligible difference in certain situations. If your commute involves a quick drive to the highway entrance ramp, 30 minutes at highway speed, and a quick drive from the exit ramp to your desination, you spend so little time actually decelerating that you won't see a consistent difference in mileage since other variables, like temperature and traffic, will make a much bigger difference.
But it can make a potentially larger difference if you spend a lot more time decelerating. Here in Chicago, I feel like I spend most of my drive slowing down. That's an exaggeration, of course, but with so much accelerating and decelerating between stoplights, I do notice that it's easier to keep my average MPG above 30 if I do a bit of planning and keep the car in gear while rolling up to a stoplight or departing Lake Shore Drive exit ramps. It's a small difference, to be sure, but it's there. And since you'll never get BETTER mileage by shifting to neutral in your DSG, there's surely no point in doing so, as it's not a normal procedure while driving an automatic.
As for the other differences you feel between your Altima and the VW, it's also due to the difference between a DSG and traditional automatic transmission, not the turbo. Your Altima has either a 4 speed automatic or a CVT, depending on how old it is, but either way, power is transmitted from the engine to the transmission though a torque converter. Basically, a torque converter uses the spinning motion of the engine's flywheel to push hydraulic fluid onto an impeller, which is connected to the transmission. There is no direct mechanical connection between the engine and the transmission while standing still in D. This allows the engine to stay running without the wheels turning. When you accelerate, a surge of hydraulic fluid flows through the torque converter and applies torque to the transmission, turning the wheels. But even while standing still, there is still some fluid flow through the converter, so when you lift off the brake, the car "creeps" forward. This is a very simplified explanation, but you get the idea.
Since the DSG is more similar to a clutch-operated manual transmission, the transmission must completely disengage from the engine when the car is stationary. The clutch doesn't engage until you command acceleration through the pedal, so there's no "creep" like in a regular automatic, and there's a slight hesitation at first press of the throttle as the transmission attempts to smoothly engage the clutch to get the car rolling. The turbo definately takes a moment to spool up, and this turbo lag certainly magnifies the delay. But in my experience, turbocharged cars with regular automatic transmissions suffer from the same delay as well.