Not sure that there's a demand for this information, but I wanted to see what the MPG difference was between coasting downhill in neutral vs keeping it in drive down the same hills, 'eco' mode, just enough throttle to keep regen from kicking in.

Basically, a comparison between 6 cylinders idling and 3 cylinders at higher rpm but otherwise virtually unloaded.

All the hills I ran this on are capable of just maintaining vehicle speed on the downslope for ~1/4 - 1/2 mile each. I maintained 60-65mph on all downhills, both in N and D so I know that even in drive, the engine was providing minimal propulsion and no regen- as close to unloaded as I can get in 3 cylinder mode.

These hills start at 15 miles into my 34 mile commute. Trip MPG was between 39-41mpg at the start of the hills each time, so these numbers come on top of some padding- they are NOT instantaneous readings.

No A/C, all 4 windows cracked 1 inch. ~85F temp, minimal winds.

I ran this 3x in each drive state- averages presented here.
15 mile point:
hill 1, in drive: gained 1.7mpg avg
hill 1, neutral: gained 3.3mpg avg

18 mile point:
hill 2, in drive: gained .7mpg avg
hill 2, neutral: gained 1.2mpg avg

23.5 mile point:
hill 3, in drive: gained 1.4mpg avg
hill 3, neutral: gained 2.5mpg avg

Nothing groundbreaking, but a little quantification of the benefits of neutral coasting downhill, as opposed to staying in gear.

 

gonavy,
your data matches my seat of the pants experience with neutral vs. eco downhill; there's only a couple of places on my commute where neutral coasting is practical and beneficial, but I do notice some mpg gain with N there. It's the N idle speed rpm that does it vs. the higher rpm in 3 cyl ECO, at least 800 rpm difference, I'm thinking.
thanks for the research data.
Lewis

 

thanks.

I just wish I could replicate this using a FAS. Unfortunately there are no spots on my normal drives that allow it to be done consistently, given the HAH tranny limitations. Well, maybe 1 spot, but I need to go late at night. Xcel is trying to do a FAS-vs-neutral comparison in his HA, which should give us good numbers to drool about, no doubt.

 

I've read that in most cars with automatic transmissions its detrimental to coast above 30-35 mph....they always reccomend towing with the drive wheels off the ground as well. Is the accord transmission different?....
hans

 

Hi Hans:

___The Accord tranny is definitely different then most Auto’s as it is a manumatic and can be shifted like a manual. Most other non-Honda automobiles have a hell of a time using this technique …

___Good Luck

___Wayne R. Gerdes
___Waynegerdes@earthlink.net

 

I don't know for sure, but...
With the engine off, the transmission fluid (which provides the hydraulic coupling in the torque converter) is not circulated as well (if at all?) throughout the system and thus not run through the cooler. So for towing, I'll buy that argument, maybe. As for engine on but high coast speed, I'm not so sure. The fluid is still getting pumped around. I'm not very well versed on the hydraulic pathways thru the tranny and where the pressure comes from, so I'm purely going on a plausibility argument here.

It wouldn't surprise me that at one point, it did cause damage in older transmissions, but like oil change intervals the old advice may or may not still be entirely valid? Dunno.

Anyone know a transmission expert who can weigh in here?

 

gonavy,
I looked a little more carefully at RPMs tonight; I'd say about 650 during a neutral coast, around 1800 RPM at 60 in ECO; about 1200 RPM difference is enough to make some MPG difference, especially if it's 6 cyl under no load.
I had some stretches of the road to myself tonight and played with some N coasts down to 40 mph, more than I usually can do because of traffic (2-lane road). Got 45 mpg over 41 miles, best yet on that stretch. I have yet to try a FAS and don't know that I will.
Lewis

 

concur w/ your RPM observations. To me it was a question of quantifying what exactly the delta was; I knew it was there.

I have, unfortunately, tried FAS on these same hills. I asked in another thread (which Xcel definitively answered for me) about the effects in the HAH. Turns out all Accords behave the way mine did in FAS: the tranny sticks itself in D3 (or at least locks out overdrive/5th) after restarting the engine while still in motion. No damage, but lots of RPMs, and the 'maint' light may blink for a few more seconds. Too bad- one less technique in our bag, although I do FAS on occasion when conditions are clear and I know I will be coming to a complete stop.

FWIW I've been getting 42-44.x for a month or two now each way on my 34 mile commute; pure 65mph highway except the 1st/last 2 miles. My town driving still needs help, as shown by my 37-38mpg tank averages. A little town driving hurts a lot when you've got 6 thirsty cylinders!

Driving before and after rushes on both sides of the workday has another benefit...40 hours in 4 days, allowing me to stay home Friday. Talk about instant 20% fuel savings! Not to mention the wife happiness factor. Go flextime!

 

In neutral there is no gear selected, but the valve body is mechanically open to high fluid pressure. In park you can rev your engine, because the valve body is closed. If you go 70Mph in neutral, then the tranny is driven by the wheels so there are internal parts spinning at high speed but the pump is driven by the engine through the TC ( idle speed ). There may not be enough pressure to keep parts from burning. The purpose of the neutral is really just to give you a break ( a second or two ) between R and D. People who revving their engine in neutral instead of park, can blow seals in the tranny. It happened before.

 

ok, but who's revving the engine in N and why would one do that? If I were in a '69 Mach 1 at a light and something fast looking pulled up next to me I might consider it, bt otherwise??