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?
More by Mark Stevenson
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