He is saying that when coasting (foot off accelerator), it is better to leave it in gear rather than idle.

The reason being at idle you're burning gas at 750rpm with cylinders firing.

Staying in gear, even though you're at say, 3000rpm, not one drop of fuel is used (or close to that). The ECU cuts the gas completely off.

Yep, what he said.

^ interesting you say that, because when I'm looking at my instant fuel gauge, and I drop it into neutral, I will go into the 40mpg+ area, but it stays in the 20s if I just let off.
Thoughts?

It goes 40+ in neutral but should go off the chart (60+) when you lift off completely.

I'll have to give this a better look tomorrow, sure would be handy to stay in gear.

Coasting in neutral consumes more fuel. In neutral, the injectors fire to keep the engine at idle, so you are burning fuel. Coasting in gear, the ECU switches to deceleration ... which happens to have an injector pulse width of zero.

True. But from my perspective, you can't always be driving it spiritedly, such as during heavy traffic.

So for those times when you can't be using all 332 ponies, why not drive to improve your fuel economy... you'll have saved fuel for those times you can push it!

let me just clear one thing out. i didnt get the z to drive like a granny, thats before breaking it in. as soon as i do im going to be flying

FYI: I'm no mechanic nor an auto engineer... just a dude using common sense and the fuel gauge read-out on my car...


I still don't understand why coasting in neutral would burn more fuel?

Are you suggesting that simply lifting OFF the gas consumes less fuel than placing the car in neutral to idle @ 750 RPM? I think there's a direct correlation between fuel consumption and RPM range - unless you're bogging the engine or loading it.

If your theory is correct, I could be driving down hill in second gear @ 6,000 RPM with my foot off the gas pedal (yet maintaining speed due to gravity & slope/grade) and save more fuel than popping it in neutral.

Very, very arguable, irrespective of however technical you may wanna get w/ injectors or widths.

Sure, there may be a point at which the speed one drives w/ the foot off the fuel pedal conserves fuel, but gearing and velocity have to be factored in and there comes a point where idling and maintaining a certain momentum (vs. losing momentum due to engine speed slowing the car) is more efficient.


Can you refute my non-technical assessment? If so, please do so in plain english.

Well, if he's right about the fuel getting cut during engine braking, then yes engine braking would save more gas than coasting. However, I find this hard to believe. Cut fuel = no more combustion. The drivetrain would simply be spinning the engine and sucking air through the pistons for compression braking, but without any ignitions happening (which means really they could cut the spark plug current off too). I suppose in theory this sounds possible, but I would think I would hear the difference between an engine that's undergoing combustion and one that isn't, and I would think the transition between the two states would be rather abrupt and noticeable. Sure seems pretty smooth to me.

I'm not saying it isn't true, I'm just saying I find it hard to believe :)

I've read here a few times that coasting in gear, gets you better mpg than dropping to neutral. Although this was a rather weird idea at first, I actually believe it. I have paid attn to that constantly moving horizontal bar graph in the Z's computer readout. It is true, when lifting off the gas and coasting in a gear, at higher speeds, the bar graph reads completely full (i.e. 60mpg or however high it goes up to); this is not true for when you drop into neutral, it shows like 40s or something.

That alone convinces me that you're better off coasting in gear.

Finally, I drive highway a lot, long trips for work. I've done this a few times, setting the cruise to 75mph. On one 200 mile trip, I averaged 27.7 mpg; on a second, slightly shorter trip, I averaged 28.5 mpg.

The mechanical noise of all the engine parts moving far exceeds any noise the actual ignition makes.

In full lift off the injectors do not fire (there may be no spark either, but I'm not sure). It matters not the gear or the RPM's as long as the engine is above idle speed. As you approach idle the injectors will begin firing again to prevent stall.

You do get great mileage in neutral as well because idle fuel burn is minimal and there are times where that is going to save more fuel than lift off if you can do it for longer. But if you're going down hill you absolutely should be in gear.

Theory and engine mechanics are one thing... real-life dynamics and physics are another. With an engine stationed @ a laboratory, I can see that an engine will conserve fuel by being in gear, however, not on our highways...

What I'm getting at is that the gear you're in makes a difference! To elaborate, lets assume we're coasting down a hill. Without using the gas pedal whatsoever, first gear will be less efficient to coast in than 2nd gear, than 3rd, gear, etc.... this is due to the ability to gain greater speeds. Even coasting in 6th gear will eventually reach the point where the car will NOT gain more speed. Popping the car in neutral in this case SHOULD result in increased speed (since the engine RPM's don't hold the car at a steady speed) and this should result in more distance covered in less time (speed). This is inherently what fuel mileage equations take into account...

So as I see it, when variables of physics are thrown into the equation, the whole concept of fuel savings needs to be reevaluated.

Someone should try this and report to us the finding:

1. Find a stretch of road (at least a mile) w/ a slight grade.
2. Warm-up car and go to the top of this "hill".
3. Reset fuel mileage gauge and immediately perform step 4.
4. Accelearte from first gear to 25 MPH at a steady rate.
5. Place car in 3rd gear and coast to bottom of "hill".
6. Record mileage info.
7. Perform steps 1 through 4.
8. Place car in 4th gear and coast to bottom of "hill".
9. Record mileage info.
10. Perform steps 1 through 4.
11. Place car in 5th gear and coast to bottom of "hill".
12. Record mileage info.
13. Perform steps 1 through 4.
14. Place car in 6th gear and coast to bottom of "hill".
15. Record mileage info.
16. Perform steps 1 through 4.
17. PLACE CAR IN NEUTRAL AND COAST TO BOTTOM OF HILL.
18. Record mileage info.


While this is not entirely scientific, it should produce consistent results. Ideally, we want to maintain consistent tire, engine oil and road temps. Same goes for the weight in the car (passengers, luggage, etc)... and if you test w/ the windows up or down, do so in every instance of this test.

If I were a betting man, I'd say Neutral will generally yield better mileage, especially as slope (and speed potential) increases! :)

Yes, the compression of the engine will slow you down, and the gear you are in will determine how much it will slow you down. You will have used the same amount of fuel getting to the bottom no matter what the gear (zero as long as the hill is steep enough) but your speed at the bottom will obviously be very different depending on how much engine braking there was. You will go further and faster in neutral which means you could potentially save more fuel that way in certain circumstances as you will have more inertia and hence wont need to re-accelerate up the next hill as much.