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Old 08-25-2008, 08:19 AM   #1
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Driving Techniques with Auto trans

Hey guys i have been trying to get better at the whole P&G thing. I also have tried shifting in to nuetral going downs hills and on straights(get like an extra two miles for every five) so it seems benificial. But i am worried that, that is really bad for my trans since it is an automatic. Also i did have a lower than average FE this last fill up... putting it in nuetral. When i switch back into drive the RPMs shoot up almost like as if i was down shifting. Would this cause for lower FE.

Also can anyone explain to me other driving techniques that could help out my automatic diesel??

Thanks
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Old 08-25-2008, 08:30 AM   #2
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Shifting into N at speed will not do damage to an automatic provided you leave the engine running. As long as the engine is running the transmission pump is running and lubricating the tranny. What you may want to do is what we call "rev matching". Take my truck for example. Traveling @ 60 MPH on flat ground the engine runs @ ~1,500 - 1,600 RPM. If I shift into N at this speed the RPM's fall to ~900 - 1,000 RPM. Before shifting back to D from N I would lightly touch the accelerator and bring the RPM's to about 1,500.

You're on the right track, and good luck. You'll get better, and tailor your driving to what works best for your vehicle. Also, if your vehicle is a 1996 or newer I highly recommend buying a Scangauge so you can monitor your FE in real time.

-Jay
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Old 08-25-2008, 09:12 AM   #3
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depending on the year (1995, I know) your car might DFCO which makes it almost useless to shift into neutral. no gas is better than less gas.

DFCO = Deceleration Fuel Cut Off. this is where when no throttle is given so the engine cuts off the injectors and the tires are actually spinning the motor. this only happens at certain conditions though. I said depends on year but I can't tell you if your car has it or not. mine doesn't but my wife's does.

also I have done some testing myself and found that neutral coast doesn't really work for me. two things that suck gas is RPMs and LOAD. at highway speeds, coasting down a hill (in gear) I will be pushing 2,000 RPMs but my load will sink into single digits. If I were to put the car in neutral, RPMs sink to 700 or so but load goes up to 25 percent.

I would recomend a scangauge but i don't think you are OBDII compatible because of the year. I know some cars had it in 1995. that may be something to check on.
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Old 08-25-2008, 09:21 AM   #4
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I looked on www.scangauge.com and it doesn't appear that a 95 Mercedes is compatible. Looking down the list it looks like there are several 96 & up Mercedes models that are not compatible.
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Old 08-25-2008, 10:03 AM   #5
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Originally Posted by BEEF View Post
no gas is better than less gas.
Depends on the amount of engine braking you get. Some cars will keep gliding for a long time and the engine will barely brake at all, but on some cars it takes a lot of "oomph" (highly technical term there) to keep the engine turning at a higher speed.

I suspect in most cases, even with DFCO, you're better off going to neutral to allow the engine revs to be slower. (Not to mention, the drivetrain is a pretty inefficient way to turn the engine).

As you said, a scangauge will be the best tool to tell.

-Bob C.
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Old 08-25-2008, 10:26 AM   #6
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Well that's kind of the beauty of the automatic trans. They're not "in gear or not" in most scenarios. Due to the way the torque converter works, there's a good chance that under coasting conditions the auto box has been told to remove pressure, allowing some engine/drivetrain seperation.

That's why on a lot of the american luxury cars from the 80's you could literally watch the RPM's drop off as you let off the gas. They weren't that great, as they didn't come back on the throttle nicely, but they were going for as quiet of a ride as possible. Modern auto transmissions exhibit the same phenomina. Letting of the gas you can watch the RPM's drop a good 500-1000RPM, maybe more in some cars. Do you really think the entire car is slowing down that fast?

Obviously an auto is as economical as a manual, but they operate very differently. Let's not forget those facts when talking about manual driving methods used on an auto.
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Old 08-25-2008, 10:49 AM   #7
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With my F250 on the straights, better to neutral coast on the P&Gs. Also better to neutral coast to the stop signs as much as possible, trying to time it so that you are under 35 mph before hitting the brakes, or going to drive for engine compression braking. I found that my diesel has too much compression braking for staying in Drive while P&G.

If I were going down steep hills for extended periods, then I'd go with staying in Drive and let the engine do the braking. If during this time you find yourself slowing down while going downhill, then I'd go back to neutral and effectively do Neutral-Drive P&G without the throttle. Diesels have much higher compression ratios than gassers, so the P&G is a tad different. Unless you can stay in Drive and coast a long ways, I'd stick with neutral coasting. My F250 can't do that.
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Old 08-25-2008, 11:02 AM   #8
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With a diesel, you can let it slog as soon as you've got the transmission in lockup or are above the TC stall speed, so probably briskish acceleration at over 2000 rpm until you get up to TC lockup speed (usually around 45mph) is best, then let it loaf along just as fast as it needs to to stay in lockup.
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Old 08-25-2008, 11:10 AM   #9
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Originally Posted by 99metro View Post
With my F250 on the straights, better to neutral coast on the P&Gs. Also better to neutral coast to the stop signs as much as possible, trying to time it so that you are under 35 mph before hitting the brakes, or going to drive for engine compression braking. I found that my diesel has too much compression braking for staying in Drive while P&G.

If I were going down steep hills for extended periods, then I'd go with staying in Drive and let the engine do the braking. If during this time you find yourself slowing down while going downhill, then I'd go back to neutral and effectively do Neutral-Drive P&G without the throttle. Diesels have much higher compression ratios than gassers, so the P&G is a tad different. Unless you can stay in Drive and coast a long ways, I'd stick with neutral coasting. My F250 can't do that.
Thanks for everyones input but i experience the same problem as his F250... my car slows way down if i dont put it into nuetral so im going to keep trying P&G with switching to neutral and see how i do on FE. I will try RPM matching cause i still feel wierd about having that boost in RPMs. So ill keep watching my FE and see if it can get me back up to my 35 or above range. I have a feelig this tank will be good its my first tank where i can actually try to hypermile it the whole time.

Also is there anything else that you have done metro to your diesel that has helped you or any other driving skills that i can try.

Again thanks for everyones help.
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Old 08-25-2008, 11:14 AM   #10
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I looked on www.scangauge.com and it doesn't appear that a 95 Mercedes is compatible. Looking down the list it looks like there are several 96 & up Mercedes models that are not compatible.
Thanks Jay for looking for me i didnt think they had it for me anyways... does the fact that it is an import matter about the laws for the scangauge port that is supposed to be required??
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