Breakpoint for P&G in N vs. DFCO

[theholycow]

Regarding the subject line: You can't really compare P&G to DFCO. You can compare neutral coasting to DFCO but neutral coasting is only one component of P&G. One person has posted that he has good results with P&DFCO with an automatic, which while possible, I believe is unlikely to work for most people.

You have to consider where all the energy is going. You turn some fuel into energy. That energy gets wasted in the cooling system, engine (pumping loss, reciprocating loss, internal friction, for each revolution), transmission, tires, etc before what remains gets a chance to get wasted on air resistance. After all the waste is done you have stored some energy in your forward motion as inertia. When you DFCO, you take up some of that inertia to keep your engine turning (pumping loss, reciprocating loss, internal friction, for each revolution). When you neutral coast you use fuel to keep your engine turning - but you turn it fewer revolutions by far.

As a general rule, DFCO is best used when you were already planning to discard that energy through braking. Neutral coasting is the better choice when you plan to maintain speed and expect to need to apply power to the road again later in order to do that.

You can, of course, experiment the usual way with A-B-A-B-A testing.

The Scangauge, reading open/closed loop, should help you get more intimately familiar with your car's DFCO behavior. Some Hondas are known to raise the DFCO floor when accessories (headlights, wipers, air conditioning) are in use, for example. My pickup truck usually has a delay before DFCO engages. My VW often refused to DFCO if I'd recently shifted gears.