Petrol vehicles have a very inefficient idle, but it's much more efficient on a diesel (one of the main reasons why real-world MPG is much better on diesels than petrols).
I was not aware of this. I know that I have been told by several people (referencing Car & Driver Mag) that it is better to keep a car in gear/under load when stopped at a traffic light because the fuel economy is better. Similarly, they say coasting in gear is better for MPG than coasting in neutral for the same reasons: cars are tuned so that a slight load on the during idle produces the best MPG. Sounds like neither strategy would ever be true with a diesel, but could be true with a gasser.
Another issue for idling is the type of transmission. A traditional automatic does create a noticeable drag on 2000 TDI Beetle when in neutral, My 2006 and now 2012 Jetta with a DSG transmission sounds exactly the same in D or N because neither clutch is engaged until I take my foot off the brake. For a manual transmission, there are two possible states: clutch in while still in gear (which is likely low drag, good MPG, but straining the clutch) or clutch out with no gear engaged (likely minimal drag, no strain on the clutch, some wear to the transmission internals that are spinning, maybe, and additional time/steps to get underway.) Any comments?
[/QUOTE]I can't think of a case where a moving car in neutral & idle would be using more fuel at that moment in time than a car which is under a significant positive load at the same speed (i.e., accelerating). I'm probably misunderstanding your question.[/QUOTE]
Is it possible that a car coasting in neutral & idle would be using more fuel than a car coasting in gear and idle? I don't see how, especially as you note a diesel is more efficient at idle. When I coast in gear with my 2012 Jetta TDI DSG I slow down faster than if I am in neutral, much faster. Being in neutral should make it so I can get more 'free distance' so to speak for the fuel I expended to attain that speed. It is difficult to see how shaving off speed faster would increase economy even if the idle in neutral is less efficient. If being in gear decelerates from 100 to 50 in .25 mi instead of .5 mi in neutral, what kind of inefficient idling could squander the extra .25 mi of coasting?
I have heard a listener call in to CarTalk (a popular car repair public radio show in the US, hilariously funny BTW) and ask about idling in gear vs neutral (with an traditional automatic presumably) and their take was that it was bad for the car because of wear and tear to the drive-train engaging and disengaging torque to the transmission, final drive and axles.
I wonder how this relates to commercial transport trucks (semi's in the US or lorries in UK) who often leave the engine idling for extended periods. Obviously they don't get better mileage since idling is 0 MPG, but maybe it keeps them warm. Never have heard a good explanation for this.
NOTE: I did find a interesting post of fuel economy here that explains some of the things discussed on this post: http://en.wikipedia.org/wiki/Fuel_economy-maximizing_behaviors