Originally Posted by DJ's left nut
I'm saying that you're looking at 1 of 2 possibilities here.
1) The car isn't going to stay at 3500 rpm - it's going to simply idle down. Unless you're referring to the second it will take for the computer to realize that you're not accelerating (and if you were in overdrive, it shouldn't even take that long), there's no practical difference there. Take your foot off the pedal and your ECU will get to work keeping your mileage up - it's a cheap and easy way for Ford, etc... to improve fuel economy without drastically changing the motor design. Now, the other possibility is what I think you're referring two and thats when you're coasting down the hill (foot not on the gas) and your RPMs have come up to keep you from going 100 mph - in other words, 'involuntary' engine breaking.
2) Again, engine breaking does not have any impact on fuel economy. You may be right, on your steeper grades in Colorado your manual might be pushing 3500 going down the hill. But that's not a true 3500 - it's an idle speed that your transmission is running up to 3500. You're not using any more fuel there than you would if you stepped on the clutch. The fuel isn't what has you at 3500 - the wheels are.
The only difference in MPG would come from the fact that by engine breaking, you're reducing the distance you travel. If it takes you 5 seconds to stop, you will have used the exact same amount of fuel over those 5 seconds as you would have if you'd have just stepped on the clutch - but if you just step on the clutch you'll still be traveling, so your MPG goes up. Presuming, however, that you're stopping for a reason, you're just going to use your brakes to stop at the same point there's no actual travel gain.
There is a very surprising lack of information about this on the net, I read the popmechanics article which basically seemed to just say it's unsafe to be in neutral. I am surprised frankly, I know nobodies lives depend on this but I would think there would a definitive test done at some point.
I am basing most of this on my personal CO driving experiences which are not typical for sure, situations where I just hit neutral (exceeding the speed limit and coasting distance certainly come into play here) and coast for miles then the road evens out and I definitely would have had to get back on the gas earlier in an auto. This is mostly what drives me crazy about now being in an automatic here.
Oh, and I was basically talking a little point 1 and 2. I know #1 savings would be minuscule but they are savings none-the-less, #2 is interesting to me as if this actually held up that neutral at 800rpm would burn more fuel than the entire drivetrain spinning at high rpm. It makes sense until you engine brake an s2000 down a mountain road at 8000 rpm and realize it might be more efficient than sitting quietly at idle at 800 rpm. Know what I mean?