Manual tranny - cruise to a stop or downshift?

[onion]

I used to be a coaster... and it's probably better for the vehicle. But once I started some occasional truck driving and learn't them non-synchronized trannys, I started downshifting just out of habit. Sometimes I even double-clutch not thinking about it- and it actually seems to help the old NP435 find 3rd gear a little easier.

 

[BearZDefect]

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Personally, I coast until the engine slows to idle speed, and pull it into neutral at that point. I sit in neutral with the clutch out until it's time to go.

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This is what I do too, don't even need to press the clutch to get it out of gear that way. I do not coast in neutral, but neither do I downshift to slow down, unless it is a long downhill where additional engine braking is good to prevents the brakes from overheating on the long stretch.

 

[JohnBrowning]

I have alway's downshifted. I was taught that this was proper way to operate a vechile. The Army used to teach this method of usage and when I went to Commercial Truck Driveing school they also taught downshifting and if you coast more then the length of the truck you fail the driveing portion in Michigan. You can not shift in an intersection either.

Now as to engine brakeing their is plenty of evidence to suggest that it helps to prevent ring's from sticking. If you know you need third gear to go 25MPH and you are in 5th gear going 45MPH why notcoast down to 35MPH and drop it in 3rd gear where you need to be any way's? Does not make much sense to take it out of 5th use the brakes to get to 25MPH then put it in third gear anyway. What if the the light turn's green while you are stopping if you have been downshifting you are already in the right gear to take off if you have not been then you are in need of a shift before you can jump on the gas!

 

[sciphi]

+1.

Even with re-learning and lots of downshifting, the clutch in my Hyundai Elantra lasted 128k miles. I still downshift to save fuel, and to help in timing lights. Being in gear has helped me more times than not.

 

[01rangerxl]

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I've learned to shift into first just before the car comes to a stop. I find it's easier for the synchronizers to engage when they are turning slowly than when the car is at a dead stop.

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I guess that varies from vehicle to vehicle. My truck shifts into first easier when at a full stop.

I usually coast to a stop. My original set of brakes was still decent at 70K miles, so I'm not too concerned about brake life.

 

[rriddle3]

On my last truck I rev matched, downshifted, and got 192k on the clutch before replacement. Never had a tranny problem with it, either.

 

[mechtech2]

Ditto Kestas - [ ^ It's way up there ^ }

 

[GeaugaFletcher]

On most modern fuel injection systems, the computer will cut fuel when RPMs are above a certain threshold (say 1500 or 2000 rpm) and the throttle plate is closed. Fuel used = zero.

Fuel used during downshifting does not equal zero.

As I've been told, downshifting to slow the car is a vestigial practice from the days of inadequate drum brakes. There's really no reason to do it in a modern car unless you feel like it.

 

[Quattro Pete]

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On most modern fuel injection systems, the computer will cut fuel when RPMs are above a certain threshold (say 1500 or 2000 rpm) and the throttle plate is closed. Fuel used = zero.

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Right, but to keep those rpms above that threshold (it actually may be even lower than 1500 on some cars), you need to have it in gear. Otherwise, if you have it in neutral, the car consumes as the fuel as quickly (in gallons per hour) as if you were standing and idling.

 

[Max_Wander]

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On most modern fuel injection systems, the computer will cut fuel when RPMs are above a certain threshold (say 1500 or 2000 rpm) and the throttle plate is closed. Fuel used = zero.

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Right, but to keep those rpms above that threshold (it actually may be even lower than 1500 on some cars), you need to have it in gear. Otherwise, if you have it in neutral, the car consumes as the fuel as quickly (in gallons per hour) as if you were standing and idling.

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+1

My coasting fuel cut is about 1200rpm. I disengage the clutch just before so as not to ruin the exhaust note aesthetics with that one second of burbling

 

[Bamaro]

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On most modern fuel injection systems, the computer will cut fuel when RPMs are above a certain threshold (say 1500 or 2000 rpm) and the throttle plate is closed. Fuel used = zero.

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Right, but to keep those rpms above that threshold (it actually may be even lower than 1500 on some cars), you need to have it in gear. Otherwise, if you have it in neutral, the car consumes as the fuel as quickly (in gallons per hour) as if you were standing and idling.

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I've always wondered about that. How do you know whether a specific car does that or not?

 

[CBDFrontier06]

Pretty much any F.I. car will either cut fuel or reduce it while idle is elevated and the throttle position reads 'closed'. I have read that the fuel supply is reduced vs. shut down in an effort to keep the catalytic converter at optimim temperature, but I have no way to confirm whether or not that was true. In either case, you're going to use less fuel by taking steps to keep idle elevated by mechanical means rather than letting the ECU feed the engine to keep it running.

 

[eljefino]

Ran my saturn stick shift on a scan tool and it cuts fuel under decelleration/coast down as described in this thread. Lengthy discussion, includes video

 

[brianl703]

One could connect a noid light to a fuel injector and see if it's glowing at all when decelerating in gear.

 

[CBDFrontier06]

Thanks for the confirm, eljefino. I do remember Honda commercials of yore touting this as one of their many fuel saving devices. I just had no concrete evidence that all carmakers employed it. I do wonder if any employ a partial shutdown, providing some fuel to keep the cats hot, or if they just rely on the hot air being pumped through while decelerating.

 

[brianl703]

They may implement a timer that turns partially turns the fuel back on when decelerating for longer than a certain amount of time.

As far as Honda touting it as "their" fuel-saving measure, the original idea probably came from Bosch.

One of the major advatanges of fuel injection is that deceleration emissions are easily controlled, unlike with carburetors.

Incidentally, I was driving on I68 in Western MD and there are hills/mountains on that highway steep enough that you can leave the car in 5th gear and start down the hill at 60MPH and be going 80MPH by the time you get to the bottom. While I was going downhill in 5th gear with my foot off the accelerator, I noticed the coolant temperature was decreasing. I think it started at 190F or so and got to 180F by the bottom of the hill.

 

[ex_MGB]

Slow down with car in whatever gear it was in when you determined the need to stop, use brakes if needed and when tach shows 900 rpm put in neutral and complete stops with the brakes.

If you're driving a sportscar on a back country road, you do a lot of downshifting. Doesn't hurt a thing, like the man said, even if you don't rev match, with no throttle there's scant wear on the clutch.

Finally, every now and then, I like to downshift and very gradually let the clutch out with some nice slippage but again, no throttle. This keeps the clutch surfaces "in tone", that is, it smooths out minor surface irregularities on the clutch surfaces.

 

[AntFZ]

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Slow down with car in whatever gear it was in when you determined the need to stop, use brakes if needed and when tach shows 900 rpm put in neutral and complete stops with the brakes.

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This, but 900 RPM may not apply to all cars. I think that's a good baseline, though. No car should be starting to stall out at that RPM...that I know of.

 

[sciphi]

My Hyundai Elantra doesn't let the RPM's drop during shifting. I'll take it out of gear, have my foot off the gas for the time I'm stabbing the clutch in, and the engine note stays the same until I'm in the new gear. When I take it out of gear, the computer keeps the engine at speed for a second. Kinda weird driving a car that doesn't keep the RPM's up. Saves fuel by having the engine accelerate less.

 

[Drew99GT]

I downshift to 4th, sometimes 3rd, both with a nice rev match and stick it in neutral right as I come to a stop light or stop sign. I usually try and do it when the rpms are down to about 2,000 as it "feels" much less strenuous on the clutch and trans. I mostly just downshift 5th to 4th coming off the highway but use 3rd if it's a steep offramp, or a light just turned yellow/red and I need to slowdown faster. 140K, original clutch, 90,000 miles on current brakes with still about 30% pad life left. I downshift a lot when driving in the mountains, you have to to save your brakes; I end up usually going slower and ---- people off as the fly by only to JAM on the breaks around corners and what not, but hey, it seems like most people like to put brakes on their cars every 30K or so! Engine braking, even with not downshifting, that a manual trans equipped vehicle provides, saves brakes AMAZINGLY!