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Going downhill in neutral--"Green" or "Goofy"?

Didn't I hear somewhere that shifting into neutral gear when you go downhill saves gas? (If so, why didn't I hear that it makes the computer go kerfluey in a car with automatic transmission...or DID it?) My 2007 Corolla's computer had to be replaced after weird death-gasps, and I'm wondering if I hastened it to an untimely demise by coasting down the hills and dales. I made it all the way from the Green Mountains down to Massachusetts in neutral (well, almost). I'm sure an NPR listener offered this as a "green tip." No?


  • edited August 2008
    Your engine is still connected to the transmission whether you're in neutral or drive. Just keep it in drive and ignore goofy tips like this. In fact even if there was a chance that fuel savings could be realized here they would be so miniscule that you wouldn't even be able to reliably observe it. You'll see more benefit from properly inflated low-rolling resistance tires and a gentle right foot.
  • edited August 2008
    Absolutely goofy. It's unsafe, and may actually use more gas (by keeping the engine running in neutral, as opposed to having the engine spun at a higher speed in gear with the injectors shut off for some cars). Your efforts will be paid off 100 times more by careful use of the gas and brakes.
  • edited August 2008
    Green? No. Goofy? Perhaps. DANGEROUS? Definitely, yes!

    Coasting downhill in neutral is not safe, and it saves ZERO gas. If your foot is off the gas pedal, the engine is getting only enough gas to maintain idle, no matter how fast the car is going.

    Coasting did not cause the computer to fail.
  • edited August 2008
    Just think of all that gas you're saving by not driving it because the ECU blew up. :P
    IF you save ANY gas, it'll be about a half teaspoon per tank. Compare the cost of that to the cost of that ECU you just replaced
  • edited August 2008
    Depending on the car, in some cars you may gain a little mileage by doing it, in others you will loose some mileage by doing it. In any case the difference is small and it also depends on the car, the speed, the grade of the hill the length of the grade the speed limit, the traffic etc. Don't bother.

    While I have not researched it, I suspect all that in and out of gear is not going to do the mechanics of the transmission any good. (Transman, care to comment). Transmissions are expensive.

    Computer: No it will not damage the computer. It may cause some minor problems with adaptive systems, but that would not damage the computer nor would it likely cause any serious problmes.
  • edited August 2008
    Many states is it is actually illegal to perform this.
  • edited August 2008
    The biggest problem here is that when you say "coasting in neutral" nearly everybody on this board assumes you are talking about freewheeling down some really long and steep mountain road and gives an answer based on that assumption. If I find myself in that situation, I keep my car in gear and let the DFCO feature cut off all fuel use. Ditto for approaching a red light when I have so much speed that I have to brake anyway, sometimes I even downshift to keep the car in DFCO longer.

    My commute to work is filled with dozens of, oh, let's not call them hills, let's call them coasting opportunities. These are grades so mild that you don't need to use your brakes when you freewheel down them. What happens when you keep your car in gear? DFCO does not come into the picture because if you take your foot off the gas, the engine braking slows you down and you need to re-accelerate. What happens is that you have to hold a small amount of throttle just so the engine will let the car coast. The fuel use difference is the difference between idle and a 3000 rpm fast idle. A Honda Fit uses about .3 gallons per hour idling, triple that idle speed and the fuel burn will be about triple the idle fuel burn.

    Is it illegal? Cool! Another good reason to do it! Stick it to the man!

    Does it save only a "teaspoon" of gas? My last tank yielded 46 mpg. How's your gas mileage?
  • edited August 2008
    Goofy and dangerous
  • edited September 2008
    I do not want to start a new blaze about saving gas versus dangerous driving, but --- in a country like Denmark where I live(highest point, all of 600 feet above sea-level) you ain't gonna see any big danger levels. I have driven my 1995 Citroen ZX 1,4 monopoint injection over 600 miles as recommended here on this forum - that means no freewheeling under any circumstances, just letting the engine slow it down. Ended up with: 35,8 mpg. The next 610 miles: Coasting at any place possible comming up to turns, junctions, downhills etc. without slowing down other traffic. This time: 42,95 mpg.

    Make your own decisions, but remember that if You are coasting down a mountain, You take other lifes in Your hands than Your own. Do You believe that that's worth two ounzes of gas.
    Best regards
  • edited September 2008

    Regarding: "Do You believe that that's worth two ounzes of gas."

    In the US, there are apparently people who do believe that endangering the lives of others is worth 2 ounces of gas. Isn't that sad?

    But, then again, this is a nation where many people seem to think that the ability to "gut" a moose qualifies one for the office of Vice-President of The U.S. That is even sadder, I think.

    We live in a very strange world.

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