Approaching a Stop--Out of Gear or Not?

Maintenance/Repairs
1

I’ve always wondered something. Let’s say you’re driving (in a car with a manual transmission) 55 MPH and approaching a red light. You could 1) take your foot off the gas pedal and coast for a while, then brake to a stop, putting the transmission in neutral right before you stop, or 2) take your foot off the gas pedal, shift into neutral, coast for a while, then brake to a stop. I tend to do #2 because the engine runs at idle speed the whole time you’re coasting, whereas with #1, it slowly runs from the RPMs it was at when you started coasting down to idle speed when you’re finally stopped and it’s in neutral. But my big question is this–in scenario #2, is gas being burned during this process? In other words, if an engine is turning over at 2,000 RPMs because the car is in 4th gear and it’s coasting along, but you’re not pressing down on the gas pedal, is it burning gas? I’m not sure I’m explaining this very well, but I hope someone understands what I’m trying to ask. Thanks!

2

when the engine is running it burns gas. In scenario 1, in older cars it did burn more gas than at idle. In modern cars with computer controlled engines, little if any additional gas is burned than would be at idle.

3

I downshift, but I dont recommend that you do that unless you match the engine and transmission rpms, so that there is no wear on the clutch as you work your way down through the gears. This takes some practice and skill. I think anyway you slow down as fine, as long as you dont coast with the clutch pedal pushed in the whole time…this causes wear and tear on the clutch throw out bearing.

4

Keep the transmission in gear until you are almost stopped. Coasting in neutral is dangerous and should not be done. If your foot is off the gas pedal, the computer is only sending enough fuel to the engine to maintain idle speed, regardless of the actual engine speed or the speed of the vehicle.

5

“Coasting in neutral is dangerous”. I’ve heard this many times and disagree! I’ve always shifted to neutral when approaching a stop, with never a problem.

And yes, i slip out of gear without the clutch, why increase clutch wear?

6

I’ve always slipped the transmission out of gear (clutch or not, easy enough to learn this procedure) and coast to the stop. Now, you have to allow for road & traffic conditions . . . I don’t coast UPHILL to a stop that much, I watch traffic and don’t coast so slowly to offend other drivers, and so on. As soon as I can, I slip out of gear to drop the RPMs to idle. My MPG is almost always better than EPA and I get great service life with my clutches. Rocketman

7

I also coast to red lights and stop signs in neutral and I never use engine braking to slow my car, ever, but then, I don’t live in the mountains.
I’m not sure if modern fuel injections shut off all fuel during coasting or not. Diesels do and some older open loop efi systems did but I think that today they keep injecting fuel to keep the catalytic converter lit. Also, catalytic converters can create NOx if there is a lot of unburned oxygen in the exhaust.

8

can’t shut off all fuel, engine would stall. Also true with diesels. Unless you mean coasting in gear.

9

Coasting in gear is what I meant. Diesels have a idle governor that makes the injectors inject whatever amount of fuel it takes to maintain a set idle speed. Anytime the engine speed is above idle and your foot is not on the accelerator, the engine is just pumping air through it.
Thus, when you drive a diesel, you actually burn more fuel if you coast in neutral.
With a gasoline engine, the situation is different. When the engine is braking, the higher vacuum behind the throttle sucks more air through the closed throttle than if the engine was idling and the carb or injectors have to spray additional fuel in order to maintain a stoichiometric fuel mixture. With a gas engine, you burn less fuel if you aren’t using the engine to brake the car.

10

thanks

11

Thanks for the replies–I appreciate it.

12 
Some diesels DO shut off all fuel when coasting in gear above idle speed.  Current VW TDI's for example. They are drive by wire cars BTW.  So it does consume slightly less fuel when coasting to leave it in gear above idle speed. 

However the difference is so small as to be meaningless.  It is also true that the safety difference is too small to worry about, except when going down long stretches requiring the use of brakes.  It is foolish there not to use engine braking.
13

The only place that I have ever heard that coasting in neutral is dangerous is at truck driving school. If you are driving a tractor-trailer coasting is dangerous because:

(1) Big trucks do not have synchronizers in their manual transmissions. Shifting into gear requires the engine to be turning as a precise speed. If you coast in neutral with a truck and find your self in a situation where you need to accelerate, it will take time to manually synchronize your engine and transmission before you can shift into gear.

(2) Using the air brakes while coasting could lead to you accidentally draining your air tanks before the compressor can refill them. Downshifting helps the compressor maintain pressure in the tanks for the air brakes.

If you are in a car, your brakes are hydraulic and your transmission has synchronizers that will allow you to shift into your needed gear in an instant. The myth that coasting in neutral is dangerous in a car has been handed down by truck drivers and their descendants.