Ask the Editor: Freewheeling From Coast to Coast

Mark Stevenson
by Mark Stevenson
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ask the editor freewheeling from coast to coast

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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6 of 62 comments
  • NickS NickS on Jul 14, 2016

    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.

    • See 1 previous
    • JimZ JimZ on Jul 14, 2016

      @TR4 correct. the front pump is driven off of the snout of the torque converter (usually,) so like you say, no engine, no line pressure.

  • Scoutdude Scoutdude on Jul 14, 2016

    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.

    • See 1 previous
    • Scoutdude Scoutdude on Jul 14, 2016

      @NickS 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.

  • Analoggrotto I'm trying to find a way to bash this vehicle using the Telluride, ATPs , AVMs, DSDs or STDs and I'm still working on it... stay tuned.
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  • EBFlex Pretty awesome this thread is almost universally against this pile of garbage. Tesla really missed the mark.
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