!

Honestly, I think the main reason is most drivers are driving on automatic pilot. Most people don't have very good situational awareness and further, they're focusing on other things than driving. When I drive, I focus on driving, and it's a painful experience for me to ride with others who do things like that (waste all that gas zooming up to a light that I saw was red four seconds ago).

 

Both. while going down the hill, the car in gear will have better gas mileage. In theory, it's infinite. But as someone said, there might be a tiny amt. of gas leaking into the cylinders.

I was talking about fuel consumption over time. I argue that over time, the one the idles in N down a large hill will have better mileage. Here's why. Let's say you are in gear going down the hill at 3000rpm. You were just robbed of some kinetic energy and used it to push the cylinders up and down at a 3000rpm pace. When you're idling, you're only using the amount of fuel it takes to maintain 600rpm.

 

I was just thinking, if you have gas leaking from an injectors because the fuel rail is pressurized, wouldn't it fill up the cylinder with gas when you go and park the car since the fuel rail retains residual pressure when you turn off the engine?? Fuel injectors are very precise pieces of equipment. Barring a mechanical failure, I doubt they would leak fuel into the cylinder as a result of the fuel line pressure and the vacuum created in the cylinder during the intake stroke.

 

Wrong. You are forgetting that MPG does NOT depend on time. Miles per gallon is a rate: distance traveled per amount of gas. If two people start at the top of a hill, Car 1 in gear and Car 2 in neutral, Car 2 will get to the bottom first but the Car 1 will use less gas.

But I see what you're trying to say, and it's what I said a couple of posts ago: practically speaking, we all want to get places reasonably quickly. Thus, if Car 1 wants to match Car 2's speed down the hill, Car 1 will have to depress the accelerator pedal to keep up, while Car 2 is not depressing the accelerator. In such a case, keeping it in gear clearly wastes gas.

 

Foobeca: Agreed. I highly doubt the injectors leak fuel despite the pressure. Fuel metering is VERY precise.

Anyhow, SR20DEN and Nismo87SE both said that coasting in gear the a/f ratio was off the charts and when it came back down to idle the a/f was richened. In the context of slowing down for a red light, the car left in gear will consume less fuel and save the brakes. It's a double bonus.

 

I don't think so; when you turn off the engine any vacuum in the cylinders is quickly pressurized (with air).

I didn't use the term "leak" and I don't think we should use it because it implies a problem, something Nissan did not intend. What I'm proposing is a controlled, tiny flow of fuel when the cylinder is on the intake stroke. It's not a leak, it's just not a "injection" of fuel. It may or not exist, but the concept I proposed is not a "leak".

 

You are both right in a sense. Foobeca is right that no fuel will leak out of the injectors unless there's an o-ring failure. And VQuick you're right by saying that if 2 cars are going down a hill and one car is in gear and one's in neutral, the car in gear will use less gas but will also take more time to get to the bottom.

Edit: Even if you dont wanna call it a leak, the FSM is clear when it says that fuel is CUT OFF. There's not much room for interpretation there...

 

Yeah, I was just driving and experiementing a couple of hours ago. When coasting in gear, once the RPMs get down to about 1200, I consistently noticed a sudden change in the sound of the engine. I think that's the ECU making the A/F mixture richer again to maintain idle.

 

I meant to say distance.

 

Someone with a MPG computer in their instrument cluster needs to do some experimenting. Go down some steep and not so steep hills in neutral once and let it coast in neutral, keep traveling a mile or so down the road. Record the MPG from the top of the hill to 1 mile afterwards. Then do the same with the car in gear.

I think it'll depend on the slope of the hill and the length of the slope to determine whether over a given distance, if N is better or in gear. It'll need to be at highway speeds.

 

that can be arranged. all 2k2+ have them

 

Holy crap, I accidentally made a very prosperous thread!

 

Yeah, where you been? Great topic, I think we've gotten somewhere but haven't got a definitive answer.

 

Read above

 

I think i'm about to eat my words. Remember how i was saying that with the ignition off, in gear, and coasting the exhaust sound would be different? Well anyway, i tried this the other night. I have a header-back exhaust, so it's pretty loud. I took the car to 3500 in 3rd gear, coasted for a little bit, then shut the motor off while still in gear. There was ZERO change in the exhaust volume and/or sound in general. Switched the ignition back on, still no change. This is just another thing that proves that what the FSM says is true. During deceleration in gear, there is ZERO fuel going to the motor until ~1500 RPM, which is when it goes into idle fuel trim.

 

Cool, I'm gonna try that. Thanks for reporting back. And I think it's more like 1200 RPM. Somebody on a Nissan board said 800-1200 RPM depending on the car.

 

D'oh!!!! I knew that...