Originally Posted by Smile-n-Nod
(1) With enough air in the tires, the car glides very well in neutral.
What's happening with the fuel cut when coasting in D vs. gliding in N? Why is it better to glide in N than to coast in D? (I'm fuzzy on some of the details.)
(2) You can shift manually when decelerating down under 30mph to keep fuel cut active until roughly 15mph.
Should I shift to a lower-number gear more quickly, or try to keep it in a higher-number gear, in order to keep fuel cut active?
(3) How can I tell when the fuel cut occurs? Will the engine RPM go to minimum (around 600, I think), or will it actually have a higher RPM because the wheels are turning the engine?
(1) When the engine is at operating temperature and the car is moving at more than 40mph, lifting your foot off the accelerator will cause the fuel injectors to be switched off within a second or two. The torque converter remains locked, so the engine continues to spin as it is still locked to the drive wheels and so the momentum of the car spins the crankshaft. As you coast in D and the car slows, keep an eye on the tachometer and as it falls below 1500, you will feel a downshift and the tach will bounce higher. The car will downshift as needed to keep engine speed over 1100-1200rpm during fuel cut.
When you shift to N, the engine idles as the car rolls ahead due to momentum. Fuel is burned to keep the engine running as it is not locked to the drive wheels.
Note: Your Elantra is not flat towable, so while shifting between N and D is fine, you will want to keep the engine running when the car is moving. According to the manual, the car can roll at speeds of up to 10mph for up to a mile while engine off, and the only time I use that is when rolling through a parking lot after potential parking and making my way down to the exit.
Regarding the "N or D?" question... Both have their own place in coasting. In D and with fuel cut, the additional drag of spinning a "dead" engine means the car slows and glide distances are shorter. Use this when heading down a hill, or when approaching slower traffic ahead and losing speed is the preferred outcome. To cover the most ground with the least loss of speed, shift to N. Although you are burning some fuel instead of "none", the car will travel a longer distance and with less fuel burned than if you just kept your foot on the pedal. This is best when you need to glide a longer distance on a level road to a stale red ahead, and just keeping a steady foot on the pedal would get you to there too soon, requiring the use of the brake.
(2) Fuel cut works perfectly when the lever is left in D. From higher speeds, it will drop out at about 30mph. At lower speeds (maybe 35mph), it will sometimes engage and then drop out at about 20mph. Using the manual mode can usually keep fuel cut engaged until about 15mph, but it usually means manually downshifting just before the tach hits 1500, and for the 2-1 shift, it should happen closer to 1800rpm. When in manual mode, there is much more engine braking, so this is only useful when you are heading into a required stop. In manual first, the low/reverse brake is engaged in the transmission, so there is no freewheeling - it feels like someone threw the anchor out the window.
(3) You wouldn't know for sure when fuel cut is engaged unless you have a ScanGage or UltraGage. With a ScanGage, the instant mpg will read 9999 and LP will show "open". With an UltraGage, the instant mpg will show 999.9 and the little oval in the upper right will be open at the top. Without a gage, you will feel fuel cut as an odd drag on the car, as if one or more tires are underinflated, accompanied by odd thumps as the car downshifts. During upshift, a "pressure control" bleed solenoid is used to make the shifts smooth. The downshifts during fuel cut are made at higher line pressure to make sure the engine keeps spinning, and it's sometimes noticable.
Fuel cut requires engine speed over 1100-1200rpm, so if the tach is below that range, the engine is "live" and sipping fuel.
There are several Elantra challenge drives posted here at CleanMPG, and the two most recent drives demonstrated that the car can beat its EPA estimate even when driven "dumb" - at the speed limit with cruise control while rolling on low tire pressure. And that is worst case. For you: Get the car set up to achieve, and toss in some basic/intermediate techniques, and then be prepared to see some mpg results that will put a smile on your face.