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9/22/2017 12:11:25 AM
Posted: 8/25/2005 1:40:53 AM EDT
Anyone else do this? My manual transmission car stays around 1800 - 2000 RPM's going 55 mph. On downhills (and for some straightaways following) I shift it into neutral, picking up speed and lowering my RPM's to about 1000. Put it back in gear when it slows back down to 55.

I figure it has to help gas mileage some. Instead of shifting into neutral, would it cause unnecessary wear on the vehicle to push the clutch the whole way to the floor in order to coast?
Link Posted: 8/25/2005 3:42:36 AM EDT

Originally Posted By DDiggler:
Anyone else do this? My manual transmission car stays around 1800 - 2000 RPM's going 55 mph. On downhills (and for some straightaways following) I shift it into neutral, picking up speed and lowering my RPM's to about 1000. Put it back in gear when it slows back down to 55.

I figure it has to help gas mileage some. Instead of shifting into neutral, would it cause unnecessary wear on the vehicle to push the clutch the whole way to the floor in order to coast?



I would have to think that for the hills that you have coasted down, you have climbed up at some time. So anything that is saved in coasting, is used during the climb
Link Posted: 8/25/2005 3:45:17 AM EDT
It's not like I can avoid going up the hill... I have to follow the road.

But you can save on the RPMs on the other side. Like I said, it's almost 1000 RPM's lower if I coast.
Link Posted: 8/25/2005 3:51:30 AM EDT
That seems like an aweful lot of effort for whatever piddly savings you're getting. Even if it cuts a 20 mpg fuel consuption in half, to 40 mpg, you're going from .05 gallons/mile to .25 gallons per mile. That saves you maybe $.07/mile? Unless you have some really huge hills that's pretty piddly savings.
Link Posted: 8/25/2005 4:23:25 AM EDT
"Coasting" is (or was - I haven't checked lately) illegal in Illinois, oddly enough.
Link Posted: 8/25/2005 12:37:19 PM EDT
the same thing is accomplished by liftin off the gas
Link Posted: 8/25/2005 3:13:09 PM EDT

Originally Posted By SG688:
"Coasting" is (or was - I haven't checked lately) illegal in Illinois, oddly enough.



still is and I freely admit to doing it...you hear me effa bee eye? come and get me!
Link Posted: 8/25/2005 5:17:32 PM EDT

Originally Posted By DDiggler:
Anyone else do this? My manual transmission car stays around 1800 - 2000 RPM's going 55 mph. On downhills (and for some straightaways following) I shift it into neutral, picking up speed and lowering my RPM's to about 1000. Put it back in gear when it slows back down to 55.

I figure it has to help gas mileage some. Instead of shifting into neutral, would it cause unnecessary wear on the vehicle to push the clutch the whole way to the floor in order to coast?



You're actually using more gas by shifting into neutral. When you're coasting, the engine is being turned by your tires - YOU'RE NOT USING ANY GAS!!

When you shift into neutral, the engine is being kept running by idling on your fuel.
Link Posted: 8/25/2005 5:50:26 PM EDT
A gauge indicating intake manifold vacuum is the easiest way to determine which type of driving behavior will save you gas.

High Vacuum = Poor Fuel Economy
Link Posted: 8/25/2005 5:57:13 PM EDT
Get a diesel. When the momentum of the vehicle is turning the engine (as in compression braking), the injector pump will deliver little or no fuel. At idle, it'll sip fuel at a ridiculously low rate too. Unlike a gas engine which requires a constant rich/lean ratio at all time.
Link Posted: 8/27/2005 6:04:47 AM EDT
From what I've read in owner's manuals, it damages the trans and engine, and you're not supposed to do that. Correct me if I'm wrong though.
Link Posted: 8/28/2005 3:22:54 PM EDT

Originally Posted By GTTacoma:

Originally Posted By DDiggler:
Anyone else do this? My manual transmission car stays around 1800 - 2000 RPM's going 55 mph. On downhills (and for some straightaways following) I shift it into neutral, picking up speed and lowering my RPM's to about 1000. Put it back in gear when it slows back down to 55.

I figure it has to help gas mileage some. Instead of shifting into neutral, would it cause unnecessary wear on the vehicle to push the clutch the whole way to the floor in order to coast?



You're actually using more gas by shifting into neutral. When you're coasting, the engine is being turned by your tires - YOU'RE NOT USING ANY GAS!!

When you shift into neutral, the engine is being kept running by idling on your fuel.



The motor would cease to run. If you let off the gas, the motor will still probably get the same amount of fuel as when it is idling. The vaccum should be very high, though.

I doubt it would do any damage, though. I bet your throwout bearing in the clutch will wear out sooner if you hold the clutch in the whole time while coasting, instead of putting it in neutral and letting off the clutch.

I'm kinda thinking out loud on the last one.

WIZZO
Link Posted: 8/28/2005 6:26:37 PM EDT
[Last Edit: 8/28/2005 6:40:35 PM EDT by GTTacoma]

Originally Posted By WIZZO_ARAKM14:

Originally Posted By GTTacoma:

You're actually using more gas by shifting into neutral. When you're coasting, the engine is being turned by your tires - YOU'RE NOT USING ANY GAS!!

When you shift into neutral, the engine is being kept running by idling on your fuel.



The motor would cease to run. If you let off the gas, the motor will still probably get the same amount of fuel as when it is idling.
WIZZO



Nope. You're wrong. The drivetrain has a direct connection to the engine with a manual transmission. The rotation of the drivetrain is keeping the engine running when you're coasting. On modern cars/trucks the CPU will completely pulls fuel UNTIL the RPMs fall below the idle level.



Link Posted: 8/28/2005 9:00:20 PM EDT
Since we're talking manual trannys, when you have the tranny in neautral, you're not driving any of the gearing inside, which in turn doesn't drive the oil pump. What you may save in gas will be minuscule compared to the cost of an accelerated rebuild interval.
Link Posted: 8/28/2005 9:17:37 PM EDT
Hmmnn.I think it so it is true.
Been a mechanic for about 30 years and most of this is new to me.
Not using any gas when the engine is being turned by the tires...that is a good one.
Must be one of the new hybrids..
High vac = BETTER fuel economy...
Link Posted: 8/29/2005 3:45:25 AM EDT

Originally Posted By Pistonman:
Hmmnn.I think it so it is true.
Been a mechanic for about 30 years and most of this is new to me.
Not using any gas when the engine is being turned by the tires...that is a good one.
Must be one of the new hybrids..
High vac = BETTER fuel economy...



You don't have a scan tool, do you?

Every manual transmission EFI car I know of will COMPLETEY STOP PULSING THE INJECTORS under hard deceleration / coasting down a hill. It's not getting any gas without the injectors pulsing.

Do this - take a short test drive with a scanner hooked up and watch the injector pulse width when you coast down long hills. I'm sure you'll already know what you'll see - considering you've been a "mechanic" for 30 years.

Link Posted: 8/29/2005 3:47:00 AM EDT

Originally Posted By MisterPX:
Since we're talking manual trannys, when you have the tranny in neautral, you're not driving any of the gearing inside, which in turn doesn't drive the oil pump. What you may save in gas will be minuscule compared to the cost of an accelerated rebuild interval.



No. The transmission doesn't have anything to do with the engine's oil pump.
Link Posted: 8/29/2005 6:28:21 AM EDT

Originally Posted By GTTacoma:

Originally Posted By MisterPX:
Since we're talking manual trannys, when you have the tranny in neautral, you're not driving any of the gearing inside, which in turn doesn't drive the oil pump. What you may save in gas will be minuscule compared to the cost of an accelerated rebuild interval.



No. The transmission doesn't have anything to do with the engine's oil pump.



I think he's talking about the internal pump inside the tranny.

WIZZO
Link Posted: 8/29/2005 9:14:46 AM EDT
Wizzo is correct. Granted, for a short coast you'll still have plenty of lubrication in the system, but a short coast isn't going to save you any noticable about of fuel either. Now, if you find a nice looooong slope like I-40 from Flagstaff to NM, then you'd have some issues .
Link Posted: 8/29/2005 9:29:03 AM EDT
Manual transmission cars have 'coasting fuel cut' (at least, every Mistubishi I had did). The coasting fuel cut RPM can vary anywhere between 1200 and 1500 RPMs, generally. That means that whenever your foot is completely off the gas (throttle = 0%) and your RPMs are above the coasting fuel cut threshold, the injectors stop firing.
Link Posted: 9/1/2005 8:46:05 AM EDT

Originally Posted By Pistonman:
Hmmnn.I think it so it is true.
Been a mechanic for about 30 years and most of this is new to me.
Not using any gas when the engine is being turned by the tires...that is a good one.
Must be one of the new hybrids..
High vac = BETTER fuel economy...



Negative.

Intake manifold vacuum is directly related to intake flow rate. The higher the flow rate, the greater the vacuum (i.e., less pressure). The only way to increase intake flow rate is to open the throttle.

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