Coasting with an automatic transmission

Discussion in 'General' started by Ec031299, Aug 25, 2016.

  1. Ec031299

    Ec031299 Member

    Hello, I have a 97 Tahoe an have been implementing some basic hyper milling techniques to make the best of what I have. Besides feather footing the accelerator, I have really enjoyed taking advantage of costing up to a light when I can. The only problem is that say I have a half mile to coast, and I leave my car in gear, the engine's braking effect takes away momentum fairly quickly compared to what neutral would. Is it bad for an automatic to be put in and out of gear (to neutral, especially since the shifter is on the "tree")? I guess another thing that could happen is accidentally ramming in into reverse!This is a good point because with the Tahoe weighing a little over 5,100 pounds, taking advantage of that much weight coasting is really beneficial. Besides the first question, are there any other things I can do to maximize mpg?( besides turning the car off, because my steering wheel has a lock that engages when the engine is off). Tires are aired up to max recommended pressure.
    Thank you
     
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  2. RedylC94

    RedylC94 Well-Known Member

    With most automatics, coasting in "N" is OK if (and only if) the engine is idling. If you're going to have to stop or slow down anyway, you might burn less fuel coasting in "D" if your truck has fuel cut.

    Someone with more knowledge of your specific transmission may provide a more authoratative response shortly.
     
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  3. 08EscapeHybrid

    08EscapeHybrid Moderator

    I have a 98 GMC K1500. The most important things to do are make sure your tires are properly inflated, and your maintenance is up to date. Letting maintenance slide is a mileage killer on these.

    Neutral gliding and fuel cutoff mode are both good techniques. Never shut off the engine for a glide as you will damage your transmission (unless you have the somewhat rare manual transmission). Driving like there's an egg under the pedal doesn't work. You languish in lower gears when you could be taking advantage of the better economy that the higher gears provide. I usually accelerate with a little less than 1/4 throttle. Not neck snapping quick, but brisk enough that I can get into the upper gears. Keep your speed down. The cruise control is your friend. If you have the opportunity to set the cruise between 45 and 55, you'll do rather well.
     
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  4. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    To clarify the previous post, if you have a long way to go, coast in neutral. If you have to slow down for a stop, use fuel cut. Even shift down a gear to get more engine braking.

    The shifter in your hand is just an instruction to the transmission, which will do the shift its own way. There should be an extra lock-out, or a push or pull or something to make it harder to hit Reverse. On my Odyssey, you can just slide the shifter up or down from N-D-3-2-1, but for reverse and park you have to pull and slide it.
     
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  5. Ec031299

    Ec031299 Member

    Ok, so you have the same 350 I do then. What is the max rpm that you take it up to? It seems to me when driving, that accelerating slowly gets into the higher gears fairly quickly. When I hit 38 mph, it kicks up and u can cruise turning over about 1200-1300 rpm.
    What exactly is fuel cut? Seems like this would only be in newer cars. If the fuel cuts off, how does the engine run? What mpg are you getting?
    Thanks
     
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  6. BillLin

    BillLin MASS: 2018 Bolt EV and 2017 Prime

    The name is quite representative of what's being done. Fuel cut has been around a long time, but I don't know when it became widespread. Infinite mpg is what you get. The car momentum back-drives the engine (logically, not actual output shaft rotational direction), which is why you don't get fuel cut benefits in neutral.
     
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  7. ksstathead

    ksstathead Moderator

    DFCO, deceleration fuel cut-off, is probably available in your vehicle. If so, the car will keep the engine spinning using the momentum of the vehicle (if in gear), until the RPMs drop too low and the computer starts sending fuel again.
    So, DFCO has zero fuel use, but you are often better off to coast in N with idle fuel consumption because you can go further.
     
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  8. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    The transmission in the Tahoe has a huge gap between first and second gear, just like the old Roto-10's used to have. The ratio change from 3.06 to 1.63 all but requires running the engine over 2000 before a shift or it risks stalling the engine when the ratio is slashed by almost half.

    If the 350 is TBI rather than port injected, there would be no fuel cut ability. If it's MFI/SFI, you would get it by shifting from below drive to 3 and keep the engine over 2000rpm. Early GM fuel cut was not elegant or too useful - it was more of an emission reduction strategy than a fuel/brake-pad saver.
     
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  9. Ec031299

    Ec031299 Member

    Ok so from what I have seen, coasting in gear will probably be the most practical. Also accelerating at a more moderate pace is more efficient so the higher gear is achieved more quickly. So here is a scenario: I make a left turn leaving school and have about a quarter mile to the next light. In these cases where you do not have too far until the next light, is there any point to accelerate quicker, because after the turn, I do not have that much momentum built up just yet, so it puts me in a place where I can't determine if giving it a little more gas to catch the light if it does turn green, or just coast at 10 miles per hour up to the light? I need help determining the happy medium here. I say this because from accelerating slowly, sometimes I almost miss more lights than I make from accelerating slower (of course this all depends on the amount of traffic at that time).I have heard some say that keeping up with the flow of traffic (accelerating) is the best, but then you would have less distance in front of you in relation to another car as well as less room to coast.
    Thank you
     
    Last edited: Aug 25, 2016
  10. BillLin

    BillLin MASS: 2018 Bolt EV and 2017 Prime

    Sorry, I can't help with the specifics you just asked.

    Do you know more about that traffic light a quarter mile away? Is it on a fixed time interval or one based on road sensors? If the latter, you can learn about "rabbit timing"... I'm not sure if that's a real name or something I've gotten accustomed to calling it. Anyway, you would let other people in front of you trigger the light change from red to green, or if you see someone coming from the orthogonal directions to the traffic light, you can guess when your green will likely turn yellow then red.

    Edit: Okay, I looked on the web and rabbit timing is indeed a well known hypermiling term.
     
    Last edited: Aug 25, 2016
  11. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    Rabbit timing aka light timing - learning the general flow of traffic and when the light will change. Some intersections near me have countdown timers on the crosswalks and they're the best for knowing that the green is indeed stale. Most others have blinking red lights to warn when the light is about to change. within about 10 seconds.
    Generally speaking, the worst thing any of us can do is step on the brake pedal. It doesn't really matter how fast you're going if you still have to press the pedal and throw away kinetic energy.
    I see it as a challenge - to learn more about my route and how to work around the obstructions and learn the patterns of traffic lights. Stay focused and get to know the road and you can get some measureable gains.
     
  12. 08EscapeHybrid

    08EscapeHybrid Moderator

    Mine is TBI, but it does have fuel cut. The parameters for fuel cut for this era 350 are RPM>1500 and no accelerator input>3 seconds. Fuel cut is cancelled when RPM<1000 or input is detected from the accelerator pedal.

    Use fuel cut when you want to slow down or stop. If you have your foot on the brake, you should always be in drive.

    I think I'm a tad bit heavier than your truck. Mine is just shy of 3 tons. I average 13-14 in town and 16-19 highway depending on how loaded I am and how fast I'm going.
     
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