Fuel Savings Transmission Neutral vs In Gear at Stop 10

[EurekaArchimedes1]

I notice this phenomenon when watching NYC cab drivers shift to neutral (auto trans) when approaching a stop. The technique is: when approaching a stop, immediately shift to neutral thus coasting to a stop and leave it in neutral while waiting at stop. Engine rev decrease while coasting and less load on engine while at stop is the purported benefit. I've read that the gas savings are close to nil, if any. One argument being that load on engine while in gear is offset by higher engine revs while in neutral. I've seen some equations bandied about but none have really nailed it for me.

"Do not disturb my circles!" - 212 BCE, Siege of Syracuse

 

[EurekaArchimedes1]

"Very occasionally the gauge drops to 0.2 GPH for a second or two. I've only noticed this on extremely hot days."

Puzzling.

"Do not disturb my circles!" - 212 BCE, Siege of Syracuse

 

[btrueblood]

Good job, DigitalGT, and a star for that post. Data (no matter how crude) beats theory any day.

 

[dieselhellfire]

i have a scanguage installed in my truck, and it allows me to view anything the ECM "sees" (eng. load, rpm, mpg, MAP, IAT, Ign. adv., etc). I was curious about this myth as well, and from what i have seen, no significant difference in fuel consumption can be obtained by shifting into neutral. Besides, in neutral the engine has to spin the flywheel, clutch, input shaft and countershaft. When clutched all that spins is the flywheel and pressure plate. Yes, it does wear on the throwout bearing, but you press and release the clutch only once, as opposed to twice when shifted into neutral. Engine load stays at a steady 4% whether clutched or idling in neutral, although I have noticed a short spike when you let out on the clutch once in neutral (the engine has to get the input and countershafts spinning again). I highly recommend the Scangauge. If your vehicle has OBD2, buy it. It displays real-time sensor outputs, tracks fuel mileage, even calculates horsepower in real-time (uses engine load and RPM).

 

[BrianPetersen]

Dieselhellfire, the terminology that you are using suggests manual transmission. That is a different ball game. A manual in neutral is essentially no different from a manual with the clutch in; the only diff is a minor amount of inertia because all that stuff is just spinning in ball bearings. An automatic, which has been the main subject of discussion in this thread, is a WHOLE different deal. The torque converter puts a significant load on the engine when its output stage is stalled (i.e. trans in gear) compared to when its output stage is freewheeling (i.e. trans in neutral).

 

[EurekaArchimedes1]

Many thanks BP. As you stated, manual vs auto trans are two completely different ball games, especially pertaining to this discussion. Thanks to DHF for the Scangauge recommendation. It looks like a reputable piece of measurement equipment from a cursory look on the web.

"Do not disturb my circles!" - 212 BCE, Siege of Syracuse