I suspect it's already been said: coasting in neutral will usually coast longer than coasting in D. Also if you have a tach you can see that idling Neutral coasting gives lower rpms than idling in Drive coasting. So for most cars without fuel cutoff, the lower rpms will give better fuel economy.
For neutral coast with engine off, auto tranny, I'd check the owners manual re. flat towing - that is, with the drive wheels on the ground in neutral.
Some manuals say it's OK, some may say never do it. Mine says up to 40 mph and 40 miles distance. So I only coast with engine off if I'm below 40 mph. Otherwise the engine is idling, coasting in neutral.
If the tranny fluid pump is driven off the tranny's input end and the engine is off while coasting, there is no fluid pressure in the system and some parts won't have the proper lube. Just my guess.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
I put my ScanGauge in this week, and I've learned one thing already. With the TC locked, I get better mileage coasting in N. In D, the rpm stay up around 1700, but in N they drop to about 800. And the SG reports about 25% better milage coasting in N. At low speeds, with TC unlocked, there is no difference in rpm or reported FE (though I can coast a little further in N)
So I've stopped slipping it into N every time I come up to a stop sign, only using N coast over about 40mph. I don't like the way my car handles N to D at highway speeds with the engine idling, so I blip the throttle before putting it back in D. This makes it much smoother.
yeah bumping the transmission like such is not worth it. I can see coming up to a stop light and throwing it in N and then back to D when you get ready to go. But going 40 so mph and throwing it back in D. You might not hurt your transmission in the short term but overtime its gonna rip it up. Plus paying for a new transmission and having it installed will waste your savings in gas. Just let off the gas and let it coast. If you have your tires psi up high enough you should achieve a pretty good glide.
When I do it at higher speeds I don't feel anything. In lower speeds between 30-35mph I feel a little thump. I should probably stop doing it at lower speeds. Please explain how it messes up the tranny. All I hear is that it is not good, but no one has posted why.
It is not advisable to shift to N while on the move as this may damage the internal parts of the device. When engaged at N, deceleration will allow the output shaft to overrun the hydraulic clutches and the gear trains of the gearbox. All hydraulic clutches at this gear will be at a disengaged position. The driving and driven plates will be separated slightly as they rub against one another, causing premature wear and tear. At D position, the governor or speeds sensor in the Electronic Control Transmission is still in operation. There will then be an automatic downshift, also known as coast downshift, to give better control of the vehicle and engine braking.