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Old 05-16-2009, 08:36 PM   #36 (permalink)
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Originally Posted by DIMS View Post
Neutral Shift for Better Fuel Economy - Popular Mechanics

Q: I have a question about fuel economy. If you are driving downhill, do you save gas by putting your transmission in Neutral and coasting, instead of having your vehicle in Drive? I think that you do but my wife seems to disagree. Can you give me the correct answer, so I can tell her that I’m “Mr. Right,” as usual?

A: That depends. The engine isn’t braking the car going downhill if the transmission is in Neutral, so economy would seem to be high. But if you think the engine is still using fuel while coasting downhill in gear, you’re laboring under a misconception. Most fuel-injected cars turn the fuel delivery completely off when you lift your foot from the accelerator. They still burn fuel when idling in Neutral, so do the math. The amount of fuel burned at idle over, say, a couple of miles of coasting downhill is small, but it’s still more than zero. So if you’re driving a modern fuel-injected car, you’re wrong.

Older, carbureted cars would suck gas through the engine while coasting in gear, even if you turned off the ignition switch. In this case you’d be right.
Who over at Popular Mechanics is writing this stuff? Nothing written so far is right or wrong, so anyone saying it is right or wrong doesn't know what they're talking about or is just guessing. For one, carbs can haz fuel cuts too. Numero dos is that unless the car can stay above the engine speed needed for the fuel cut, it'll more or less idle in gear, and even if the driver accelerates above the fuel cut every time they drop below it then they would have to compare the fuel usage to whatever was used when it idled down the hill. In short, it depends on the car, weather, hill, etc... So the only correct response w/o more information would be "That depends." with all the other erroneous stuff left out.
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