WEPSMAN
Champion Author
South Dakota
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Message Posted: Jan 30, 2011 7:59:15 AM
I have a car with a 5 speed. I downshift when coming up to a stop to slow the car down and save on the brakes.
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Houckster
Champion Author
Atlanta
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Message Posted: Jan 29, 2011 11:15:00 AM
Taking the transmission out of gear when going down hill has no benefit whether it be a automatic or a manual transmission. When the engine senses that the accelerator position is at minimum, only enough fuel is fed to the engine to keep it running or the fuel is cut off entirely and the momentum of the car in gear keeps the engine running.
There is a wikipedia article on this issue and the applicable text is:
"The alternative to acceleration and braking is coasting, i.e. gliding along without propulsion. Coasting is an efficient means of slowing down, because kinetic energy is dissipated as aerodynamic drag and rolling resistance, which must always be overcome by the vehicle during travel. When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required.[24]"
Fuel Maximizing Behaviors
[Edited by: Houckster at 1/29/2011 12:16:11 PM EST]
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JamCoope
Rookie Author
Maryland
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Message Posted: Jan 29, 2011 9:08:54 AM
I understand what you wrote, 2khawk, but it doesn't look like you read what I wrote.
The theory is that when going down hill and putting the car in a low gear that it isn't gas burning that is making the engine "race", but rather the momentum of the vehicle. Again, instead of gas burning and making the engine go, in turn making the wheels go; you have the wheels going (from the gravitational pull of going downhill) and THEY are making the engine go (and, theoretically, using less gas than if you were idling in neutral down the hill).
I'm not making this claim, I've just heard this thrown out there as a theory. If this doesn't apply to modern cars, is there a chance that it did apply to older engines?
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dontuknowOH
Champion Author
Ohio
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Message Posted: Jan 29, 2011 9:01:02 AM
Momentum is to ones advantage if proceeding can be accomplished,and if not,down shift(engine drag/braking while at idle kills off this advantage).
To regain MOM... extra fuel is needed,back on the go pedel,more to accel... if quick responce is needed,transmission downshifts RPM > up = more fuel required per ground distance covered! Nothing new here.
The new 2010...yr.models probably don't waste any fuel on idle position,in gear decellerating compared to early year models.? Internal combustion temps cool down under hood temps drop somewhat but still nominal operating temps.are maintained,cooling fans rest a bit(depends).
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2khawk
Champion Author
Iowa
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Message Posted: Jan 28, 2011 11:08:21 PM
Think about this. If you shift to nuetral and coast you will get the best mileage because your not working your engine. Most cars are direct fuel injected these days and an engine that is running 2500rpms is going to use more fuel than one that is running 600rpms. Down shifting also slows your car much faster so you will have to accelerate sooner than if you were coasting.
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