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Old 06-23-2008, 07:47 PM   #1
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Question Downshift override?

I've been on a road trip with my family in my wife's minivan - a 2005 Town & Country Limited - which is a bit of a pig, though not terminally wretched (averaging around 24 MPG on the trip, fully loaded with 2 adults and 2 kids in car seats, plus clothes and beach trip paraphernalia, 90-95% highway).

While playing around with the cruise control, I thought that it would be really helpful if I had a switch within easy reach that would override the auto transmission and keep it from downshifting on uphills. I found (predictably) that my MPG improved the longer I was able to keep the revs lower in overdrive, but the only way to accomplish that on uphills with an auto (without a Tiptronic or some similar improvement) is to kill the cruise and let the speed drop on the ascent. In an MT car, I'd just keep it in fifth and downshift only if things start to get ridiculous. I'd like to have the override switch handy, so that I could trip and hold it at the bottom of a hill and release it at the top or before then if the speed drops too much.

Anyone know if this has been discussed at some point?
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Old 06-23-2008, 09:53 PM   #2
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A lot of people seem to think the cruise control is the bottom line on how to get good gas mileage. However, that's only true for lazy people with twitchy feet or delerium tremens. There is a better way to get good mileage with an automatic transmission and it's real simple. Just stop messing with the accelerator!

If you like to cruise at 60mph, when you come to a grade just keep your foot at the same position you had it at doing 60 on the flats. You're car will slow down, it will downshift when it needs to and upshift when you crest the top. Have some patience and you will get almost the best mileage that you can squeeze out of a car designed to waste fuel for gas-guzzling spendthrifts who want to go the same speed up, down or flat.

The trick is to find the point on the accelerator where the downshift occurs and drops into the higher ratio gear without forcing excessive, fuel-wasting high rpms. You can only achieve this with a light foot. This will certainly be slower than you want to go, being used to going fast, but it will be close to the optimum mileage.

And you can make up for lost time by going really, really fast down the other side by letting gravity do the work, not the accelerator. Screaming passengers are a verification of healthy hypermiling techniques. The extra momentum helps you attack the next hill without using gasoline. Don't coast in neutral, though. Doing that can make the transmission pump run backwards and burn up. You'll wonder where 4th or 3rd gear went. That's hell on mileage.

I can tell you are thinking that going uphill in low ratio gears is giving you better mileage but this is counter-intuitive. This takes a lot of fuel which is not being burned as efficiently as it would be at higher rpms with significantly less load. In other words, you're moving away from the stochiometric mix by forcing too much fuel onto your available air by running at low ratio up a grade. There is no free lunch.

But by allowing the transmission to receive a constant low gasfeed rpm and translate that, according to its shifting program, into variable speed over hills is the key to improved mileage over uneven terrain.

Auto transmission shifting programming is more agressive and wasteful the more your foot is into the accelerator, and it is more efficient and thrifty the lighter you use your foot. Moving your foot changes the program, be it software or hydraulic. Again, for the best mileage, keep your accelerator pedal at the same position you choose to cruise at on level terrain.

My rule of thumb for determing the optimum stochiometric mix for any engine is finding the point on the torque curve where power begins to level out. You will want to hold your rpm just prior to that for best mileage. Optimum torque will reveal itself on grades while using a light, constant accelerator position. Any engine will pull comfortably on a constant grade in the correct gear at a certain accelerator setting. You want to find the point where the engine will pull without straining at a low to moderate rev. More gas pedeal will move you out of thrifty torque and into horsepower, which is wasteful. Most people drive in the horsepower realm. They waste gas. Truck drivers always drive in the torque realm. If you dirve like a truckdriver, you'll get better mileage.
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Old 06-24-2008, 03:10 AM   #3
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You don't want to let any Chrysler product with an ultradrive transmission slog, ever. It would actually be wise to downshift it sooner when well loaded.
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Old 06-24-2008, 05:19 AM   #4
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ptero, you have a way with words!

"Screaming passengers are a verification of healthy hypermiling techniques."
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Old 06-24-2008, 08:17 AM   #5
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I've been considering getting a custom tune for my truck that would make it shift way lower, using torque and low RPMs for everything. Another strategy I considered was hacking the wires that go from the computer to the tranny, which are labeled on a diagram such that I think I could get better control of what gear it's in while being able to use more of the gas pedal.

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Originally Posted by ptero View Post
Don't coast in neutral, though. Doing that can make the transmission pump run backwards and burn up. You'll wonder where 4th or 3rd gear went. That's hell on mileage.
Can you elaborate on this?
- Why would the pump run backwards?
- How would it make you wonder what happened to 4th and 3rd?
- How would it decrease mileage?
- Is it specific to the Chrysler tranny?

I've been coasting in neutral in my GMC and it has greatly increased mileage.
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Old 06-24-2008, 01:05 PM   #6
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ptero, thank you for the very comprehensive response. I think there are and will continue to be times during a family road trip when becoming one with my drivetrain as your post would seem to require is more of an aspirational goal than a realistic one, however I certainly will keep your advice in mind.

I also share theholycow's question regarding this part of your post:

Quote:
Originally Posted by ptero View Post
Don't coast in neutral, though. Doing that can make the transmission pump run backwards and burn up. You'll wonder where 4th or 3rd gear went. That's hell on mileage.
If you could elaborate, I would appreciate it.
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Old 06-24-2008, 11:03 PM   #7
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I did neutral coasting in my Eagle, which is Chrysler branded and it still drives perfectly fine. For a gas guzzling, full sized, automatic transmission rust bucket, I really miss driving that car daily.
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Old 06-25-2008, 08:04 AM   #8
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Quote:
Originally Posted by atomicradish View Post
I did neutral coasting in my Eagle, which is Chrysler branded and it still drives perfectly fine. For a gas guzzling, full sized, automatic transmission rust bucket, I really miss driving that car daily.
How many miles did you put on it while using that practice?
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Old 06-25-2008, 08:15 AM   #9
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The recent plethora of posts about "coasting in "N" is bad" is interesting.

Anybody who claims this care to pony up some technical information on the inner workings of transmissions to support this argument?
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Old 06-25-2008, 08:53 AM   #10
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Way back in the mists of time there were some transmissions where the pump did not run in neutral.
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