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10-22-2012, 11:56 AM | #8 | |
Sauntering Vaguely Downwards
Join Date: Sep 2005
Location: Columbia, Mo
Casino cash: $2799099
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Quote:
1) The car isn't going to stay at 3500 rpm - it's going to simply idle down. Unless you're referring to the second it will take for the computer to realize that you're not accelerating (and if you were in overdrive, it shouldn't even take that long), there's no practical difference there. Take your foot off the pedal and your ECU will get to work keeping your mileage up - it's a cheap and easy way for Ford, etc... to improve fuel economy without drastically changing the motor design. Now, the other possibility is what I think you're referring two and thats when you're coasting down the hill (foot not on the gas) and your RPMs have come up to keep you from going 100 mph - in other words, 'involuntary' engine breaking. 2) Again, engine breaking does not have any impact on fuel economy. You may be right, on your steeper grades in Colorado your manual might be pushing 3500 going down the hill. But that's not a true 3500 - it's an idle speed that your transmission is running up to 3500. You're not using any more fuel there than you would if you stepped on the clutch. The fuel isn't what has you at 3500 - the wheels are. The only difference in MPG would come from the fact that by engine breaking, you're reducing the distance you travel. If it takes you 5 seconds to stop, you will have used the exact same amount of fuel over those 5 seconds as you would have if you'd have just stepped on the clutch - but if you just step on the clutch you'll still be traveling, so your MPG goes up. Presuming, however, that you're stopping for a reason, you're just going to use your brakes to stop at the same point there's no actual travel gain.
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