Pulse & Glide or In 5th Gear
 

I'm travelling 70kph, the road is flat, traffic can be either thin or thick.

I can either pulse and glide between 55 & 70kph, or get it to 70 in 5th gear, and not go hard on the pedal (maintain steady throttle position).

An Idling car wastes fuel. In fact, you are better off in gear going down hills, as your car will cut fuel, as it also will in gear and no throttle.

If in neutral, it will still use fuel, but with the engine unloaded (in neutral), the car will glide further - using fuel to idle as it glides.

Which is better, why?

I used to sit in neutral, until I found that if in the right gear, no fuel is used as opposed to Idle fuel in some downhill runs (But there's obviously a choice to be made depending on conditions I guess - in some cases, neutral could work out much better than fuel cut off, or is that the more dominant case, which is why I see so many ecomodder's swearing by Pulse & Glide?)

I spend a lot of time on the highway.
I live in a high traffic area.
If I were to pulse and glide, I would turn the car off in glide mode, but P&G is not an option here, I would get rundown. I take a lot of heat from other drivers for holding at 55 as it is.

In my Paseo, I P&G with the engine off while I'm gliding. This gets me my 60+ mpg that you see in my fuel logs. Without that, just cruising in top gear its very hard to get close to 50 mpg.

I almost never coast down hills in gear. If I know I'm going to gain a ton of speed going down a hill, I make sure to crest the hill at a lower speed so as not to be going too fast by the bottom of the hill. If I'm unfamiliar with the area and start going too fast down a hill I will put it in 5th gear to slow the car and also cut fuel. However, that is not as good as neutral coasting.

A motionless car idling wastes fuel. A car idling at cruising speed is awesome.
Braking wastes fuel. If you're in gear going down hills, you are braking.

I've got a poll question:
What would you rather be doing?
1. Having your engine constantly burning gas under load.
2. Being under load as little as possible and the rest of the time only burning enough gas to keep it turning over.

With a petrol car, P&G should be more efficient.

Accelerate at the BSFC - it'll be at a fairly high load

True, and that's where the major gain is with P&G : the engine gets shutdown on the glide.

But you'd be (engine) braking.

That means this technique is less efficient, unless you actually need the braking action to keep your speed within legal or comfort limits.

My car consumes about 0.5 L/H.
Doing 100 kph, I'm seeing 0.5 L/100km or 470 mpg.
If I do 50 kph, I'm getting 1L/100km or 235mpg.

It'd be stupid to pass that up !

Under 15kph, I'm getting too close to my target fuel economy (4 L/100km - 58.8 mpg) for it to be worthwhile.

Coasting with the engine on is ultimately more efficient than rolling in (any) gear, and having the engine slow you down.

It's just a matter of properly matching your initial speed to the length of the glide.

I was an avid user of engine braking, getting good - not great - mileage out of any car I've driven.
Coasting engine-on improved my mileage considerably.

What the others say. I get much better fuel economy P&G than in gear. I usually glide NICE-on, only turning the engine off on long downhills.

I thought about this some more and really believe it's up to the conditions at the time you have to make the decision.

There's a hill that's relatively large and winding near me, it's easy to chuck it in neutral and you'd probably reach 90 - 100kph by the bottom of it, giving you a heap of momentum to make it up the other side. It's a fun stretch of road.
It's got road work on it now, so it's speed limited to 40 (or somewhere between that and 60 if you look at the 'others').

I thought about that stretch and concluded that engine off coasting would not be as fuel efficient as chucking it in second gear and 3000 RPM all the way down to maintain 40kph.
Why is that more fuel efficient (using fuel cut instead of engine off coasting)?
Because you'd have to restart the engine at the end of the coast :).

Pulse and Glide has it's place though, nice straights of road with PnG between 55 & 70, or similar bands, you do get the average speed, and on dual lane carriage ways, it works a treat as others may pass if they get annoyed with your speed 'band' selection (realistically, it's still close to the speed limit, so it's actually pretty good).

As I indicated above, to the conditions, identifying fuel cut situations which might work in combination with PnG is probably where perfection can be achieved.

Sure, you have to use the best technique for a given situation.

Usually, the best way to slow down is (from best to worst)

coasting engine off
coasting engine on
engine braking with regen (mild and full hybrids)
engine braking without regen
braking (friction)


When going downhill, coasting may lead to excessive speed, so engine braking is the technique to use (best with regen) on steeper slopes.
Mild slopes can sustain coasting - or help your car run in top gear without lugging the engine

My P&G technique: P&G and highest gear
 

For what it's worth, here's my P&G technique which allows me to P&G AND stay in my highest gear (6th for me). Keep in mind that I have a big (vastly overpowered) diesel pickup, so my technique may or may not apply to others.

1) Do the pulse. How fast I go can depend on conditions (traffic & grade), but usually I go to about 45-50 mph (72-80 kmph).
2) Put it in neutral and coast (glide) down to about 30 mph (48 kph). For me this ground speed corresponds to an engine speed of abot 900 rpm. This is just a little bit over my idle speed of 750 rpm.
3) Pulse again (in 6th gear).

This allows me to put it back in 6th gear at the end of the pulse without having to have my engine "jerk" back up to a significantly higher engine speed than idle. Keep in mind, however, that I'm driving an overpowered diesel with tons of low-end torque (at 1,000 rpm I have about 500 lb-ft/678 N-m of torque). This allows me to run such low rpms without hitting the torque curve.

I can't vote either way, I do both equally depending on the circumstances - terrain, lights, traffic, speed.