By on July 14, 2016

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B. Breckenfeld writes:

My cars from the early 2000s had automatic transmissions that seemed to allow freewheeling when you lifted off the pedal. I used this for better gas mileage by letting the car coast when I could see red lights far ahead.

Starting about 2010, my cars produced by GM began acting more like their standard transmission counterparts by employing engine braking and downshifting as they came to a stop. This works great in mountain driving where engine braking is needed, but wouldn’t freewheeling on flatter ground allow better gas mileage?

I’ve also noticed that my brakes now last close to 100,000 miles due to less braking needed in city driving.

Why have the engineers changed this? Has there been any real world testing that shows automatic engine braking gives better overall results than freewheeling?

There’s no better way to improve fuel economy than cutting off fuel delivery during the stop-and-go nature of city driving by way of start-stop ignition engines and hybrid cars. But there are also smaller strategies an automaker can implement to improve fuel economy.

Keeping an engine engaged while cutting off fuel to said engine is one of those strategies.

“Many manufacturers employ the same mechanisms when the vehicle coasts to a stop, which includes cutting fuel delivery when the vehicle is coasting,” explained Chris Day, senior manager of Powertrain Performance at Nissan Technical Center North America. “Each manufacturer employs its own strategies when it comes to fuel consumption, which can cause the variance that the letter writer is speaking about. The engine stays engaged with the transmission to keep the engine rotating and able to operate at 100 percent when requested.”

With older vehicles, freewheels made it possible to smooth shifts and bring the engine back to idle when drivers throttled off. The reason behind this strategy was simple: old engines are loud and letting the engine return to idle made them a lot quieter. The alternative was to keep the engine running at drivetrain speed. Freewheeling transmissions tapered off in the 1960s as transmission technology improved, but GM continued using freewheels in its three-speed THM400 automatic transmission, which you could find in GM trucks until 1991. (Fun fact: The THM400 was also used in the Ferrari 400i, Jaguar XJS/XJ12, and multiple Bentleys and Rolls-Royces between 1968 and 1990.)

But when it comes to modern cars, most of the “drag” you feel from the engine comes from these fuel saving strategies — at least as far as I can tell. What say you, B&B?

Mark Stevenson is the managing editor of The Truth About Cars. He is easily swayed into buying vehicles from brands that no longer exist. You can find him on Twitter and Facebook.

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62 Comments on “Ask the Editor: Freewheeling From Coast to Coast...”


  • avatar
    gtemnykh

    I think the “rule of thumb” for in-gear coasting fuel cutoff is somewhere around 1200rpm, judging from what my ScanGauge II outputs on several mid-late 90s cars. As far as transmission downshifting goes, that does infact seem to vary a lot among different manufacturers and eras of automatics. Perhaps the strategy is that by downshifting, they can keep the RPMs up as speeds drop, which can in turn allow for a bit more fuel injector cutoff as you roll up to the light.

    • 0 avatar
      dal20402

      It’s substantially higher than that in my LS460, at least judging by the instant fuel economy gauge. I have to shift down to a gear where the engine is spinning somewhere between 2000-2500 rpm for the computer to cut off fuel to the engine. I’m guessing that’s for smoother tip-in if you get back on the gas in a higher gear.

      Perhaps not coincidentally, the LS460 has the “freewheeling” feel the author is missing. The car will glide forever and a half without slowing down much. Sometimes it still surprises me and I end up having to brake a bit harder than expected.

  • avatar
    NormSV650

    DFCO OE deceleration fuel cut off. This can be adjusted on GM cars with HPTuners software and a licensed VIN through them. Rpm/speed engagement and release points can be changed or I just leave them as stock settings.

  • avatar
    RedRocket

    You have the GM transmission acronym incorrect. It is supposed to be THM400, not TMH. THM standing for “Turbo Hydra-Matic”.

  • avatar
    maserchist

    I don’t know how many transmissions, if any, still incorporate an output shaft driven “rear oil pump”. This obsolete Powerglide component was what permitted “push starting” a pre 1967 PG car. It also was the part which permitted my PG governor to explode when hydraulic pressure exceeded whatever the design limit were. That was a result of hanging a 1st gear until ~70 mph was reached. It certainly surprised more than a few stick shift jockeys back in the day, that a 283, of all engines, could run quickly. In a 4 door Chevelle 300 ! Unfortunately, the 1-2 shift put the motor completely out of any powerband I thought I had…

    • 0 avatar

      I don’t think any tranny introduced after 1965 had a rear oil pump – which is why they can’t be towed with rear wheels on the ground unless the driveshaft is disconnected. (Or today, why FWD must be towed w/drive wheels off the ground and AWD must be flatbedded.)

      • 0 avatar
        JimZ

        didn’t Volvo have one for a while longer?

      • 0 avatar
        toplessFC3Sman

        I know many front wheel drive cars that can be flat-towed; I think it was a selling point for Saturns at the time. I’ve seen a lot of Chevy Equinox’es being flat-towed too

        • 0 avatar
          Drzhivago138

          Saturns can be flat-towed because the auto trans keeps circulating fluid even when in Neutral, IIRC. 95%+ of all vehicles I see flat-towed behind motorhomes are some Saturn model–if they’re not Wranglers or Libertys, that is.

          • 0 avatar
            statikboy

            Does anyone know where to find a comprehensive list of automatic vehicles which can be flat-towed? I’ve searched online, but any references I’ve found seem to cite specific cars only, ie: Saturn SL. Searching for specific models hasn’t been successful either.

          • 0 avatar
            DenverMike

            edmunds.com/car-buying/what-cars-can-be-flat-towed-behind-an-rv.html

  • avatar

    I think there’s a comma out of place somewhere in the article but I’d have to re-read it and that’s not something I’m willing to do.

  • avatar
    cdotson

    Lockup torque converters have become more prevalent over the past 20 years, with more automakers preferring to lock them up earlier and keep them locked. In the 90s it seemed more common for lockup only to occur over 45 mph or so and to disengage during coasting.

    Deceleration fuel cutoff does aid fuel economy but increases engine braking. Given the increased weight of many vehicles and a public tendency to brake too little too late this seems less like a tradeoff and more like a win-win and that probably factored into a wider adoption of increased engine braking.

    While not ideal for dedicated hypermilers I’m willing to bet the average motorist comes out ahead.

    BTW, my Ram 1500’s front brakes were replaced at 155k with serviceable life remaining and the rears were replaced at 190k because I had trouble keeping the parking brake tight. It’s a 5-spd and I engine brake aggressively and even hypermiled it when gas was above $3.

  • avatar
    dividebytube

    When driving stick, if I see a red light up ahead I push the clutch in and coast up to the intersection. Often leaving the car out of gear. My wife, on the other hand, will downshift and engine brake… only to push the clutch in at the last moment.

    I once uh, tried to hypermill a ’01 Mercury Marquis – putting the transmission into neutral and coasting up to a red light / stop sign. Also slow takeoffs. I did see a few mpgs in improvement, but I’m sure my fellow drivers weren’t too happy (or surprised!) by an old man car coasting ahead of them.

    I never understood many drivers method of speeding up to the red light and then hitting the brakes at the last moment.

    • 0 avatar
      JimZ

      “I never understood many drivers method of speeding up to the red light and then hitting the brakes at the last moment.”

      they’re the same people who can’t figure out why they get poor fuel economy in their Ecoboosts.

    • 0 avatar
      FormerFF

      When I’m driving a manual transmission car, when approaching a stop I leave the car in gear and coast until the engine reaches somewhere around 1200 RPM to get the most engine braking I can without downshifting, then I drop it into neutral and coast and brake to a stop. Downshifting during deceleration means you’re bringing the engine back up to driveline speed with either with the clutch, which is uncomfortable and creates clutch wear, or with the throttle, which means you are wasting gasoline.

      • 0 avatar
        FormerFF

        I should add I sold the manual transmission car a couple of years ago. Now I’m driving a plug in hybrid, so I have regenerative braking. 4.5 miles of the 12.2 miles I drove this morning were powered by the energy recovered while slowing.

        In suburban driving, regen is a huge energy saver.

        • 0 avatar
          Carlson Fan

          “In suburban driving, regen is a huge energy saver.”

          Wish my Volt had the paddles on the steering wheel for more adjustment versus versus the shifter that offers just two settings.

          I mentioned this the other day that in the city an EV/hybrid is so much more efficient than any ICE vehicle could ever hope to be. And way nicer to drive. With the weather we have in MN right now and the AC off I bet I could get 50-60 miles out of a charge with my Volt pretty easy if I never left the streets of Minneapolis. With the AC on I could still probably get close to 50.

      • 0 avatar
        dal20402

        This is pretty much exactly my approach. (Although I don’t have any manual cars anymore.)

      • 0 avatar
        SunnyvaleCA

        I agree with FormerFF. Coast in gear to slow down. When engine drops below some point where the fuel is no longer cut (1200 RPM sounds about right in my case too), coast to a stop in neutral. Downshifting is either going to wear the clutch or use extra fuel.

        In my car, I can feel the point where fuel cutoff ceases. It’s easier to feel if you are in a low gear when coasting down. (i.e.: you only got up to 30 MPH to begin with and so are coasting down in 3rd) As you coast down, you can feel at a certain point that the engine isn’t lugging as much. I suppose if you had a binary type variable valve timing, you could have a similar feeling.

        I have a realtime ODB II scanner so can see the shift in instantaneous mileage. I’ll go from getting 100+ MPG while coasting in gear to maybe only 40 MPG. Then if I depress the clutch I’ll increase to 60 MPG.

    • 0 avatar
      NickS

      @dividebytube — when coming to a red light, you are better off leaving it in whatever gear you are, and applying the brakes until engine rpm goes down to about 1100 or so (essentially, just above engine idle speed). Then shift to neutral and use the brakes to stop.

      Downshifting to a stop might have some value on very long runs, but it invariably uses the clutch like a brake (brake pad R&R is cheaper than clutch R&R), and isn’t very comfortable either.

      • 0 avatar
        dividebytube

        I suppose this is a bad habit I picked up when my dad threw the keys of his 200k mile 5-speed Stanza at me and said “here you go!”

        I had to learn stick by myself. I was so afraid of stalling the engine that I stabbed the clutch the same time I hit the brakes.

    • 0 avatar
      SaulTigh

      I can’t figure out the people that practically tear your rear bumper off to get by you on the interstate, so they can tuck into the back of the solid pack of cars that are 4-5 car lengths in front of you. I chalk it up as one of many, many reasons I think most people suck.

      • 0 avatar
        MQHokie

        It’s because A: you’re leaving them plenty of space to merge in front of you and B: 99% of the time that “solid” pack of cars is led by a left-lane squatter running at the posted limit or barely over it, and the only way to eventually get by that car is to pass all the other ones 1 or 2 at a time.

        Perhaps you might want to move to the right when you’re not actually passing anybody.

  • avatar
    JimZ

    “Why have the engineers changed this?”

    because burning fuel while coasting is wasteful and stupid.

    “Has there been any real world testing that shows automatic engine braking gives better overall results than freewheeling?”

    because shutting off the fuel flow while coasting doesn’t burn fuel uselessly, and “freewheeling” does.

    • 0 avatar
      SP

      @JimZ, that doesn’t sound like a complete answer. I believe the questioner is referring to the practice of unlocking the TCC during coast-down and allowing the engine to return to idle or near idle. The amount of fuel burned to keep the engine from stalling for a few seconds at a time seems very small. There aren’t many roads where you can coast much longer than that. (Mountain descents aside.) Also, the effects of the engine braking caused by leaving the TCC locked would result in the car slowing down more, and having to exit the coast-down state sooner. That would result in increased fuel usage in the period of time that was converted from coast-down to cruise.

      At a minimum, this seems like a delicate balance, with a very small gain in fuel economy to be gained either way.

      Point being, your rather curt answer does little to satisfy the inquisitive mind.

      Can you expand on it to make it more useful?

  • avatar
    PrincipalDan

    I remember how much speed the old 3+3 cab 400 V8 THM400 equipped 3/4 ton Chevy would gain going downhill if you didn’t ride the brakes. It was quite scary with the truck camper on it, loaded down with camping gear unless you manually downshifted or almost burned out your brakes.

    I’m happier with the way things are now.

    • 0 avatar
      JimZ

      imagine doing that in an even older vehicle, as your 4 wheel drum brakes overheated and faded into uselessness.

      • 0 avatar
        PrincipalDan

        I was fortunate that the experiences were in the Midwest where the hills are not quite as steep. Driving a U-haul out west as a 25 year old kid I learned the limits of those brakes pretty quickly. My rental was unfortunately HD 350 V8 Gas and freewheeling automatic.

      • 0 avatar
        CobraJet

        We found that out in our 64 Impala with Powerglide coming down Pikes Peak one time back in the 60’s. Low range provided no engine braking at all so my Dad had to ride the brakes hard until there was really no stopping ability left. At the mid-point ranger station there was a brake check of sorts. Temp guns were non-existent so the ranger would reach behind front wheel to touch something. If it was the brake drum he probably drew back a finger with no skin. Anyway the ranger said pull over and park it for one hour, which my Dad gladly did.

  • avatar
    MrGreenMan

    GM used to be one of the greats with their competency at transmissions. They have sadly lost the plot on that. Perhaps they were only good at RWD transmissions?

    • 0 avatar
      olddavid

      In 1968 we drilled out some valve bodies on an old Hydramatic, making it a manual shift for all gears. Installed in a 1955 Chevy 2dr. wagon with a 327 and dual quads and 4.11 gears, it would pop the front wheels off of the ground about 12 inches. Fastest street car in our area for about 3 years, only dethroned by a 440 Super Bee. When gas is 25 cents, the sky was the limit. We sure had fun. When did they lose their ju-ju on autoboxes?

    • 0 avatar
      dividebytube

      I once bought a well used and abused ’81 Malibu that someone had put a 305 in and replaced the transmission with a TH350.

      One of the best transmissions I’ve ever had in any car – worked flawlessly and had perfect shifts. Other autos (4-speeds) I had – Honda and Toyota – seemed crude by comparison.

    • 0 avatar
      dal20402

      Their FWD 4-speed automatics are excellent. The 6-speeds, less so. Looking forward to seeing how they’ll do with the next generation of FWD transmissions.

      • 0 avatar
        Zarba

        My 09 Lucerne 3.9 has a 4-spd autobox. Good tranny, smooth, and it seems to freewheel a lot, esp vs. my 01 Acura. It gains a LOT of speed downhill.

        Only downside to the 4-spd auto is the huge gap in the tranny gears means the 2-3 shift often results in dropping out of the power band; it sometimes ends up shifting back to 2nd.

    • 0 avatar
      HotPotato

      Not just RWD. The Saturn 4-speed autobox (the later model that “learned” your driving style) read and responded to your intentions almost before you’d consciously processed them yourself. Truly a joy to drive. GM automatics were brilliant in general right up until the 6-speed in the Malibu, which upshifted early and often.

  • avatar
    cgjeep

    Brake pads are cheaper than clutches and transmissions. When I drive manuals I don’t engine brake through the gears to a stop.

    • 0 avatar
      Scoutdude

      Shifting between gears, at least when it is done properly and not during a full throttle run doesn’t cause any real wear on the clutch. It is the slipping when you get the vehicle rolling that causes the wear. So downshifting for engine braking that keeps the engine above the min rpm for DFCO will save you money on fuel and brakes w/o wearing out the clutch sooner.

  • avatar

    Every GM automatic I’ve owned over the past 20 years has had great engine braking due to lockup torque converters providing defacto direct drive and engine braking like a stick.

    I wasn’t aware anyone was building a vehicle w/o locking torque converter after about 1995.

  • avatar
    SCE to AUX

    Hybrid or EV regenerative braking is a wonderful analog to engine braking, except that instead of saving fuel, you’re actually putting some back in.

    Beside the instant torque of an electric motor, regen is one of the joys of driving such vehicles.

    • 0 avatar
      Carlson Fan

      +1

      I’ll add that the aluminum wheels on my 2013 Volt with a little over 40K miles look like absolutely brand new due to regenerative braking. No brake dust to crud them up!

    • 0 avatar
      dal20402

      The lack of regen is the single thing that makes gas cars feel the most primitive after you spend some time driving an EV or hybrid. It’s like “All this wonderful energy is just being turned into waste heat?”

      Heading down some of the bigger hills in our area, riding the regen, I can actually put a mile or two of range back into our C-Max’s battery. We drive regularly to a big park 7 miles and about 400 feet uphill from our house, and sometimes I use no energy net on the way back.

      • 0 avatar
        JimZ

        “The lack of regen is the single thing that makes gas cars feel the most primitive after you spend some time driving an EV or hybrid. It’s like “All this wonderful energy is just being turned into waste heat?””

        yep. which is why it’s kind of sad that every “mild” hybrid has flopped.

        • 0 avatar
          Scoutdude

          The problem is the “mild” hybrids had very little regen braking capability. The size of your motor/generator does matter when regen braking is concerned. It also doesn’t provide much savings on acceleration again because size of the MG matters. With a mild Hybrid you get most of the cost of a Hybrid system with less of the benefits of a Hybrid system.

          • 0 avatar
            dal20402

            This. With hybrids, you’re taking a weight penalty no matter what. Using a small motor makes no sense.

      • 0 avatar
        KixStart

        dal: “All this wonderful energy is just being turned into waste heat?”

        A thought that has not occurred to about 99.9997% of the driving population.

        JimZ: “yep. which is why it’s kind of sad that every “mild” hybrid has flopped.”

        I don’t know of any “mild” hybrid that really delivered any significant fuel savings, as compared to the extra cost. I’ve run into two people who bought the original Malibu hybrid and they were sorry they wasted the money. And the driving experience may have been less than stellar. Jack Baruths’ review of a mild hybrid Malibu was hilarious and not in a way that made me want to buy the car.

      • 0 avatar
        Carlson Fan

        “The lack of regen is the single thing that makes gas cars feel the most primitive after you spend some time driving an EV or hybrid.”

        It’s almost painful now to hit the brakes on my two “planet killers” after owning the Volt. But as others before me have said my driving style with both of them has changed since getting it.

  • avatar
    NickS

    The way I understand it is that if the transmission has a rear pump, you can push-start the car, more or less the same way as a stick. That apparently was the case with really old automatic transmissions.

    But this is not true of most every auto trans today. I believe *theoretically* they can be used to push-start, but you need to reach some really impractical speed like 50 mph.

    • 0 avatar
      TR4

      “I believe *theoretically* they can be used to push-start, but you need to reach some really impractical speed like 50 mph.”

      I seriously doubt that. If the front pump is not turning there is no hydraulic pressure. If there is no pressure, the transmission will effectively be in neutral as all the bands and clutches are released. If it’s in neutral, there is no coupling from the driveshaft(s) to the engine.

  • avatar
    Scoutdude

    Sorry but “freewheels” in automatic transmissions died long before the TH400 did. The lack of engine braking is considered unsafe.

    The difference is as noted DFCO, Deceleration Fuel Cut Off and now commonly ADFCO or Agressive DFCO. That of course is a fuel saving strategy as well as an emissions strategy. Engine braking with a closed throttle is hard to control emissons so many cars before they had DFCO would not allow the idle control valve to return to a hot idle position until the vehicle speed was very low. This was also done to prevent stalling and allow for a smoother transition if the driver suddenly got back on the throttle.

    As mentioned now it is all about DFCO. So now if the vehicle speed and engine rpm is high enough The idle control valve does close to the hot idle position and fuel is completely shut off until the engine rpm or vehicle speed gets low enough.

    Additionally more agressive torque converter lock up strategies have also increase the sensation of engine braking. In the old days they would only lock up the converter in top gear and above a certain speed and now some will lock up as soon as 2nd gear and at a much lower speed too.

    So yeah for all you hypermilers out there kicking the vehicle into neutral when you come up to a stop wastes fuel it does not save it, especially on a manual trans vehicle that is more likely to have an ADFCO strategy and hold that untl a lower rpm or speed than an automatic.

    • 0 avatar
      NickS

      I can’t say for all makes across the board, but on modern gassers idle control is done via the throttle plate (it’s DBW).

      Shifting to neutral to come to a stop is not a real hypermiling technique. True hypermiling is all about putting the engine into fuel-cutoff as much as possible, except when power is needed, and then running it only where it is at its most efficient. If you have a set of BSFC graphs it’s quite possible to get impressive mpgs, but those are hard to get. They often shut off the engine to coast in neutral if they just want to keep going. In terms of tediousness and erratic driving it’s right up there.

      • 0 avatar
        Scoutdude

        Yeah I should have said hyper miler wannabes.

        And yes on DBW cars the throttle is the idle control actuator. I bet you’ll find that the idle control program only is capable of calculating and commanding a position that is a small percentage of the total throttle range that is applicable for actual idle control.

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