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Old 11-29-2007, 05:28 PM   #1
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downshifting to achieve fuel cut

do you guys with autos do this? I just started a job driving a chrysler 4cyl with the automatic, delivering flowers. i have been downshifting under situations where i need to slow down anyway to 2nd to achieve the fuel cut. I became aware of this driving a jeep grand cherokee down a mountain in vermont. when i clicked the o/d off mpg jumped up to 99, and when i put it back in o/d, it went down to 45. this is with foot off the gas. so i figure when you dont want momentum its better to encourage the fuel cut. I do this with my vx too, going into different gears depending on how much i want to slow down. I figure the chrysler minivans are built the same as the jeep 4.0 l 6 cyl with auto, in that they fuel cut only above a certain rpm. In fact in the minivan you can feel it i think. it seems to fuel cut above 50 mph no problem, but that is not really that big of a help because aero drag slows you down so quickly that the time spent in fuel cut is short. but on long city decents, you can downshift to 2nd and keep fuel cut for a long time and save the brakes!
anyone else do this? Maybe it creates a little more wear on the tranny, but with this van, it doesnt matter, it will only be on the road another 6 months or so.
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Old 11-29-2007, 11:09 PM   #2
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I'd believe the instant mpg meter. just make sure you change the trans fluid before it goes bye bye. some of those chrysler van trannies are strong as a crystal bowl I've heard. or maybe not since you aren't planning on keeping it. never hurts and $15 for a box of fluid is cheap insurance against premature car shopping
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Old 11-30-2007, 01:18 PM   #3
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I started doing it a couple of months ago; still refining my technique. As long as the downshifting isn't overly aggressive, I don't expect any additional wear on the tranny... after all, it downshifts on its own anyway. All I'm doing is asking it to do so a few hundred RPMs earlier.

And if VW didn't want me mucking about with the shift points they shouldn't have given me an unrestrained shift lever.

The Cabrio A/T seems to be geared oddly. There's only a tiny difference between 3rd and 4th (just a few hundred RPM), then a much bigger difference from 2nd to 3rd. Forget about first... if the thing had 4WD I think I could drive up the side of the Cheops Pyramid. ) Is that a normal setup? I'd think you'd want a little more power from "passing gear" (or more overdrive-ishness in top gear).

Interesting side note: The Cabrio doesn't take the transmission into account for DFCO -- if you're not calling for fuel and the RPMs are above the threshold, the injectors cut off... even in neutral.

Rick
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Old 12-10-2007, 12:20 PM   #4
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The thing i have noticed with autos and fuel cuts is it will only turn off the fuel when the torque convertor is locked up. In my old Lincoln, this would only happen if the transmission WAS in overdrive! It would be interesting to see if you can find where your TC locks up, or perhaps go that step further and put a switch on it, as some one else did on this site some time ago.
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Old 12-10-2007, 02:45 PM   #5
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Quote:
Originally Posted by red91sit View Post
It would be interesting to see if you can find where your TC locks up, or perhaps go that step further and put a switch on it, as some one else did on this site some time ago.
If that was directed at me: As far as I know, my TC doesn't lock up.

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Old 12-12-2007, 06:22 PM   #6
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Just remember, when it's in gear and you're essentially coasting with fuel cut, your coast will be shorter due to the car's forward momentum is being used up to keep the engine spinning. If you have to slow down anyway, of course there's no loss. And 99 or infinitely high mpg certainly is good.

Oh, I wish my car was a standard... Unfortunately conversion is both time consuming and costly.
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Old 12-14-2007, 05:29 PM   #7
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I dont have an instant fe meter on the van since it is a base model. The owner of the shop where i work said that he just changed the fluid. The van only locks up above 37mph, you can feel it happen... but i am pretty sure the mpg would be higher with the lower gear selected. Now i wish i had tried downshifting to 2nd with the jeep to see what results showed up on the fe meter. anyone with an auto and fe meter wanna try it out?
I only downshift when i am going to need to stop anyway or need to keep a low speed on a steep grade. Otherwise i always conserve momentum. I know that the vx gets much better mpg when you downshift to achieve the fuel cut...It even has a special pcv valve to reduce engine drag, and with the tall gearing you can keep it in fuel cut and not lose too much momentum. I still only do it when i am going to need to slow down.
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Old 01-17-2008, 11:54 AM   #8
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I wouldn't shift an automatic, don't do it.
Even clicking the O/D button while driving isn't so hot.
If you have an O/D button select one or the other before you start off, then leave it in place. Yes, de/selecting O/D at a traffic light would be fine, but not while rolling.

Depending on the age of the vehicle you can get away with it however long, a few to 6 months, sometimes a year or two, maybe longer but it is only a matter of time, one day that transmission will start to act stupid.
So put it in D and leave it be, my attitude.

btw your FE gauge is fooled by what you're doing, my bmw does the same thing if I rev it at a standstill when it comes off 6,000 rpm's the FE gauge jumps to 45mpg now it's just a computer issue because nobody is telling me I'm getting no fuel mileage while standing.

So you're not getting better mpg at 4,000 rpm's vs. 1200 without gas, your FE is whatever it is at speed with no gas, regardless of rpm's, I think brake pads are still cheaper.

I will say letting off the gas earlier would help.
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Old 01-17-2008, 12:53 PM   #9
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what kills transmissions is overheating and not changing the fluid. changing gears is perfectly OK unless the transmission is already within an inch of it's death or you have some crappy *** trans to start with. Especially OD to 3rd. Just remember to change the fluid according to the schedule and you'll be fine.

Get a 20 year old toyota A340E or derivative and you can drag race 400 hp without mods and it'll last years. I know people that have.
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Old 01-17-2008, 03:36 PM   #10
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Well, you can tell me it's ok, but I ain't buyin it. Every time that transmission shifts gears, their is wear on the clutch plates inside the transmission. Automatic R&R is a pain and whatever your fuel savings, it isn't worth it. Brake pads are far cheaper and easier.
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