Coasting- N vs. in gear coasting
 

I have been watching this closely and on one off ramp about 1/2 mile long I have been testing. It is a long slight uphill, sweeping turn and then a long downhill. I go up on cruise control at 54mph and just before the top I switch it off. When I shift to N the rpms drop but settle at about 1300 and the instant mpg display show 70-80 88- 90 for a split second and then back into the 80s. I do seem to coase a little further and faster.
When I switch off the cruise and coast in D I see the mpg go right up and to 99 and stay there for a while and maybe I lose 3-5 mph at the end.
It seems that while in D I have DFCO and while in N is bumps the rpm to speed because it will not drop to 700 until I come to a complete stop.
When I am stopped at a light I do shift into N and this seems to hepl FE.

Good observations. What is your car?

Certainly if you plan on slowing, DFCO beats neutral.

1999 Dodge Grandcaravan ES with the 3.8l v6 4 speed auto. With conscious efforts I an averaging 27.1mpg per tankful. I accelerate slowly, drive 30mph max in the city, 53-55mph on the highway, use cruise control at every chance- it does have a 30mph lower limit, and coast as much and as far as I can. More importantly I don't let my heavy footed gas and brake wife drive this car. When she was driving it exclusively it would get 17.5-18mpg! I get 27mpg.

On the occasions that I take the highways into Philadelphia, there is a strech of down hill about 1 mile long.
Most traffic travel at 65+mph at the top of the hill, I put my car in neutral when I get to the top at 60+ mph, by the time I reach the bottom of the hill I'm going well over 70mph that even with my foot on the brakes.
Once the car coasts to normal highway speed I put the car back into "drive" gear.

I try to duplicate that everytime I travel on the highway with my car and everywhere I go. My fiancee thinks I'm nuts putting the car in neatral while traveling at highway speed.

Cat- do you have an auto or 5 speed. It makes mental sense that this should be better but with computer controlled EFI without a scangauge you will not know which is more efficent- coasting in N or coasting in D.

I have a 1989 Toyota Camry with automatic transmission and EFI. Engine rpm drops below 1000 when I put in N, returns to around 2000 when I put it back into D to resume cruising speed.
I used to have a early 90's Mazda 323 with automatic that I could also do the cruising downhill with N, the the shifting back into D was not as smooth as the Camry.

Neutral and in gear coasting varies by car. On my 1988 Escort I get better mileage coasting in gear and on my 1997 Escort I get better in neutral. Both are manual transmissions one is a 1.9L 4 speed and the other is a 2.0L 5 speed. In the '88 coasting in gear the vacuum gauge reads about 24-25 at 50-55mph in fourth gear and in neutral it drops to about 20. Apparently the ECU on the '97 doesn't receive vacuum information, because I don't get a reading of vacuum pressure on the scan gauge.

Neutral
 

Neutral coast will not registar as high mpg on the scan gauge, but the increased distance and speed will ALWAYS make up for it. Neutral coast is best.

In my opinion, in gear is not coasting at all. It's engine braking. It's useful if you have to slow down, but not otherwise. Neutral coasting is better in all other cases.

To answer the original situation: I would use in-gear fuel cut for an off ramp that goes to a stop.

That sounds most logical to me. If your coming to a stop coast as long as possible and use DFCO to kill the last of your speed.
if you are just going down a hill with no intent on stopping, coast in N only. DO NOT use DFCO. Yes it cuts off all fuel, but it's basically a jake brake, not letting you coast as far or as fast.