Do you coast in gear or put in neutral on your manual transmission?

[southtj]

I heard something interesting recently and if true then I've been doing it all wrong. Usually when I come up to a light or see that I'll be stopping soon, I'd put the transmission in neutral and just coast to a stop in the hopes of saving a few pennies of gas. But now I'm hearing that you should leave it in a gear if you want to save even more fuel. What I'm hearing is that if you take your foot off the gas and leave it in gear then the computer will completely cut fuel off to the engine saving more gas than if you put it in neutral. In neutral, gas has to continue to flow to keep the engine running.

Does anyone know if the computer shuts off the flow of fuel when coasting in gear on a manual transmission Jeep?

 

[greg92jeepxj]

I won't speak from fact , but I don't think a PCM will shut off fuel flow upon deceleration auto or manual. A PCM will keep the fuel system pressurized accordingly. If you accelerate , the demand for more fuel to satisfy the increase in air entering the engine is the same as the adjustment made accordingly for deceleration. Fuel trim is controlled from various means on an electronic sequential F/I system . The idle air control will open as needed upon deceleration to prevent stall as will the injectors will fall back to its pre programmed fuel map at a specific idle speed. The only difference I have ever seen between auto and manual is the actual idle speed , that is with carburetored engines. Electronic injection will maintain idle whether auto or manual.
Decelerating to a stop in gear , clutch out until rpms fall too much is keeping the engine spinning faster due to compression braking while downshifting which may possibly use more fuel due to maintaining idle but there are other factors to consider. Saving gas is the least of your concern. Coasting in neutral is not the best way to treat your transmission . The internal shafts and bearings were meant to work in unison for lubrication purposes . If you select neutral , the main shaft is spinning at road speed but the countershaft is not . A countershaft spins at engine rpm . Manual transmissions mainly lubricate by spinning gears splashing lube to moving parts , there are no oil pumps in manuals. It is best to downshift as you would up shift , rpm and road speed accordingly. If our transmissions were not synchromesh , we would find out fast that we are doing it all wrong. I realize your question was pertains to fuel consumption and not gear shifting , but only an engine at idle will save gas and coasting in neutral in my opinion is penny wise dollar foolish. Holding the clutch in too long has its own faults. Just more excess wear and tear. When I sold my '78 jeep cherokee , the Borg & beck clutch was as strong as I bought the truck . Well over 100,00 mi. and factory original.
I never held the clutch in too long. Off the light , but not waiting at a red. Slowing , I just braked until road speed dictated downshifting . Maybe this is just my way of driving , but I did save the clutch and never had a trans issue. Gas mileage , well , it was a 360 2bbl. and the truck weighed over 4000 lbs. not much you can do about that in stock form .

 

[southtj]

Good write-up, thanks Greg. So...stay out of neutral unless stopped, downshift as needed and foot off the clutch when stopped.

My TJ that I had for 9 years I had to replace the throw out bearing after about 80,000 miles. I think it was because every time I was sitting still I had my foot on the clutch the whole time. Not doing that on this one.

 

[greg92jeepxj]

Your welcome southj . I remember in yellow school buses and in Ryder rental trucks you would always find the yellow sticker " do not ride the clutch " . Almost everything is automatic nowadays but yes we must remember any excessive contact with the clutch pedal WILL cause excessive wear and ultimately premature failure of the throw out bearing. An inexpensive part that requires a complete teardown to replace . Even when I was just slowing for a curve , I would just brake with the clutch out until downshifting was necessary. Of course if you intend to brake to a stop , you will hold the clutch in until you stop and shift to neutral and release clutch until ready to place back into gear. Sorry for the AAA auto school lesson , its just that it is the only way I know to clarify. Lots of luck and good health to you and your jeep ! Greg

 

[greg92jeepxj]

Good write-up, thanks Greg. So...stay out of neutral unless stopped, downshift as needed and foot off the clutch when stopped.

My TJ that I had for 9 years I had to replace the throw out bearing after about 80,000 miles. I think it was because every time I was sitting still I had my foot on the clutch the whole time. Not doing that on this one.

I remember Terry Mason posted something that was for placing your left foot on while idle for jeeps. It was intended for auto trans , but I don't see why it cannot be used for manual trans ?
would be a great idea for just the purpose we posted of.