Gm 6t40 - Fuel Economy, Hypermiling, EcoModding News and Forum

BrandonMods · 05-23-2014, 01:22 AM

Hi all. I have a 2013 Chevy cruze with the GM 6T40 automatic transmission. My question is, can shifting into neutral from drive damage the transmission whilst doing an engine on coast? The owners manual is very vague about it and only says,

"Shifting out of P (Park) or N (Neutral) with the engine running at high speed may damage the transmission. The repairs would not be covered by the vehicle warranty. Be sure the engine is not running at high speed when shifting the vehicle."

Doing an engine on coast has been very beneficial for fuel economy on this car, but I would rather not jeopardize having saved money from increased fuel economy and only having to spend about 2 grand to buy another transmission. That being said, if I do shift back into gear its is usually going around 25 to 30mph. Would any of you say that this qualifies for a "high speed" shift back into gear from neutral. Thanks in advance for your responses and advice

dirtydave · 05-23-2014, 08:21 AM

What does it say about flat towing?

user removed · 05-23-2014, 08:22 AM

I do it constantly in my 2011 Fiesta but it's a salvage rebuilt car with no warranty. If I was in your position and had a warranty I would only do what the manufacturer specifically approved in order to maintain the warranty.
You have to assume the worst, which in my case would be a used transmission, which in your case would be a free new or rebuilt transmission.
Once your warranty has expired then you can choose, as I have, to coast in neutral, engine on. You can see the mileage in my fuel log over the last 24k miles.

regards
Mech

user removed · 05-23-2014, 08:25 AM

It's my understanding that new cars have the ability to detect things like neutral coasting at higher speeds. Right now with GM recalling more cars than they have sold since the bailout, I'd be careful to preserve the warranty status.

regards
Mech

elhigh · 05-23-2014, 08:49 AM

The way that statement is written, it looks to me like they're talking about dropping it into gear at high revs [I]while at a standstill[I], like dumping the clutch. If you're already moving then the transmission's shafts are already turning at something close to the right speed and the shock should be minimal, are virtually nil if you're good at matching engine revs to ground speed.

You'll need to do a little experimentation. I have one car whose tranny is definitely not a "smart" tranny; put it into N for a coast and when you get back into gear, it defaults to 1st every time. VERY exciting, and not in a good way at 60mph. So try it out at modest speeds first until you're confident of your machine's behavior.

gone-ot · 05-23-2014, 02:22 PM

As long as the engine is operating, you're OK, because that transmission only has a "front" hydraulic pump, meaning fluid is only moving when the input shaft from the engine is turning, turning that pump. Shut off the engine and the pump stops, stopping hydraulic flow within the transmission...and simultaneously stopping both lubrication and cooling.

We HAD a 2011 Cruze...but got rid of it.

Eastwood · 05-23-2014, 04:17 PM

Hey Brandon,

I have the same car as you, tires and everything, just a different color.

Lacking a true manual, I've been doing engine on coasting as well. I always try to match revs to where it is while in gear and it's always been very smooth and unnoticeable when going back into drive even on speeds from 30 - 50 mph.

Like you've heard here, I've also heard both sides in other places. One says it's a bad idea and one says that it's okay since we have a newer, smarter car. Use your best judgement, I guess.

Quote:

Originally Posted by Old Tele man

We HAD a 2011 Cruze...but got rid of it.

What prompted the change?

gone-ot · 05-23-2014, 05:14 PM

1) GM doing nothing about ALL the problems.
2) Lousy city & highway milage (2011 3.87:1 ratio lowered to 3.53:1 in 2012)
3) Local GM-Goodwrenchs = BADwrenchs.
4) Nice idea, but lousy parts & service.
5) GM overall disdain for customer problems.

BrandonMods · 05-28-2014, 01:26 AM

Just to update I have decided only to shift into neutral when I know ill be coming up to a stop (trust me I keep a close eye on the flow of traffic) and also when I know ill be shifting back into drive when going at low speeds (around 15 mph or lower). Regarding flat towing it says to do it in neutral. Also the transmission does not do a hard shift into a lower gear at higher speeds it actually does go into the gear that the transmission should be at the current vehicle speed (i.e. usually at around 43 mph the transmission is in 6th ) I have done very minimal experimentation at higher speeds and shifting from neutral into drive at 45 puts me in 6th gear. So apparently the transmission is 'smart' and does go into the proper gear without any jerking or major shift shock. As for the quality of the vehicle... I probably should have bought a used TDI. I like this car as it is my first brand new car but that being said it will probably be my last new car for a very long time. Im working extra to pay this thing off then I plan on recouping on it and buying a TDI to be able to use bio-diesel.