Nope...there are lots of ways to explain it, but the best I've seen is:

http://auto.howstuffworks.com/question477.htm - copied in part below

What speed should I drive to get maximum fuel efficiency?

This is actually a pretty complicated question. What you are asking is what constant speed will give the best mileage. We won't talk about stops and starts. We'll assume you are going on a very long highway trip and want to know what speed will give you the best mileage. We'll start by discussing how much power it takes to push the car down the road.
The power to push a car down the road varies with the speed the car is traveling. The power required follows an equation of the following form:

road load power = av + bv2 + cv3

The letter v represents the velocity of the car, and the letters a, b and c represent three different constants:

• The a component comes mostly from the rolling resistance of the tires, and friction in the car's components, like drag from the brake pads, or friction in the wheel bearings.

• The b component also comes from friction in components, and from the rolling resistance in the tires. But it also comes from the power used by the various pumps in the car.

• The c component comes mostly from things that affect aerodynamic drag like the frontal area, drag coefficient and density of the air.

These constants will be different for every car. But the bottom line is, if you double your speed, this equation says that you will increase the power required by much more than double. A hypothetical medium sized SUV that requires 20 horsepower at 50 mph might require 100 horsepower at 100 mph.
You can also see from the equation that if the velocity v is 0, the power required is also 0. If the velocity is very small then the power required is also very small. So you might be thinking that you would get the best mileage at a really slow speed like 1 mph.
But there is something going on in the engine that eliminates this theory. If your car is going 0 mph your engine is still running. Just to keep the cylinders moving and the various fans, pumps and generators running consumes a certain amount of fuel. And depending on how many accessories (such as headlights and air conditioning) you have running, your car will use even more fuel.
So even when the car is sitting still it uses quite a lot of fuel. Cars get the very worst mileage at 0 mph; they use gasoline but don't cover any miles. When you put the car in drive and start moving at say 1 mph, the car uses only a tiny bit more fuel, because the road load is very small at 1 mph. At this speed the car uses about the same amount of fuel, but it went 1 mile in an hour. This represents a dramatic increase in mileage. Now if the car goes 2 mph, again it uses only a tiny bit more fuel, but goes twice as far. The mileage almost doubled!

Efficiency of an Engine

In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.
This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.

Speed Equation Result
3 mph 3+32+33 39
2 mph 2+22+23 14
Power Increase 25
51 mph 51+512+513 135,303
50 mph 50+502+503 127,550
Power Increase 7,753

You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.
So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:

• Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.

• Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.

• Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.

In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
If you drive your car in the "sweet spot" you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.

 

That's because an engine is more efficient at WOT.

 

I don't care if the engine uses no fuel at 1500+ while coasting with your foot off the pedal.

I can get farther in neutral (and hence engine at 800rpm idle) because the resistance of the engine is not slowing me down.

I will get more miles by idling and coasting in neutral, than coasting in gear and then having to apply throttle every now and then to keep the car going and above 1500 rpm.

 

At lower speeds I can barely tell the difference in resistance between 5th or 6th and neutral.

 

OK, this establishes that 40-60 mph allows you to travel the furthest on a tank of gas but it doesn't say wether it's better to maintain 40mph at say 1800 RPMs in sixth gear or at 2500 RPMs in fourth gear, or atleast how you'd go about determining which is the best.

I sense that maintaining 40 MPHs with only 1800 RPMs doesn't require me to depress the gas as much and that the car has a tendency to roll farther because of the lesser engine resistance when I lift off the gas. Obviously if I come up to an incline the engine won't have enough power and stall out but on a flat surface the low RPMs aren't a problem.

 

I can guarantee you can go noticeably farther in neutral than while in 6th gear.

 

Obviously you'll go farther, but I don't think you can say for sure that you burn less gas idle coasting 500 yards than you do keeping it in six and giving it whatever gas it might need to reach 500 yards.

 

Then you basically contradicted yourself with this post and your previous one when you said you said you can barely tell the difference between 6th and neutral.

It's noticeable and how you can tell is how far you can coast. It's a drastic difference.

 

Isn't there a mechanism in the engine that shuts off the fuel injector when there's negative load? Ie, coasting in 6th gear = no fuel while coasting in neutral = same amount of fuel used in idling.

 

I'm not sure what you're getting at but you seem more interested in protecting your driving habbits than how much fuel you are or aren't using.

 

I've made similar changes to my driving style but I'm up from 22 MPG to 29 MPG in my 6MT TL

 

Muncher, see post number 38 in this thread.

On the 6th vs. neutral. You two (not you, Muncher) are going at two different things.

Hypermileage coasting includes the use of long neutral coasts with either idling engine or (in either advanced or stupid modes, depending on your take) the engine off. In this technique, leaving the transmission engaged and getting the fuel cutoff is actually counterproductive due to the additional drag from the windmilling engine and accessories.

The advantage of in gear deceleration is the fuel cutoff vs. idle fuel use. The idea is that you lift at a point sufficiently far from the stopping/delay point to minimize your braking. The avoidance of idle fuel use is not prudent if the delay is minimal, however if you are approaching a known stopping point or stagnent traffic , it is a very good plan.

On the other hand, if you are on a highway with lights, and you know the timing, it is often worth it to coast at idle, so as to conserve speed (actually momentum, but that's another matter) if you know you won't have to brake.

 

"advanced or stupid" LOL

I've read those hypermileage sites and they are VERY impressive in terms of overall MPG (IIRC >150mpg), but yeah, I'm not sure I would feel comfortable shutting the engine down on a long coast. I thought hybrids would to this automatically? Also if instead of a starter motor you've got a substantial electric traction motor between the gas engine and the transmission (i.e. hybrid) it's probably less of a concern.

I suppose since I've got a 6MT I could very easily put it in neutral, shut off the motor, and coast, and when I needed to run again I'd just put it back into a (high) gear, flip the key back into the on position, and slowly let out the clutch. Still would not be very comfortable with this ... and the long arm of the law isn't either.

 

gas sucks so bad right now i hate it.

 

Sorry, I didn't mean to come off as confrontational. I was merely pointing out that you made 2 contradictory statements.
I apologize if it came off that way.

I'm content with my driving habits and my fuel mileage. I get about 29 mpg with 70% hwy, and up to 35+ mpg on pure hwy.

 

That's exactly my point. You get farther by coasting in neutral.
The gains made in coasting in gear are lost when you have to apply throttle to get to the same point you would have if you had coasted in neutral.

I typically coast as far as I can in gear, until about 1500 rpm, and then I slip it into neutral the rest of the way.

 

Your two statements are contradictory ...

Actually, I think we mean in two different situations. If you are trying to coast maximun distance, out of gear is the choice. In the city/suburban area, in gear coasting seems better because of the fuel cutoff feature.

Examples:

Downhill - Because engine braking will cost more momentum, either technique could be right. Steeper hills, in gear allows no fuel use and speed control...shallower hills, out of gear allows maximization of momentum with minimal fuel useage.

Highway, Long Distance - Likely that out of gear is the better choice.

Highway, Suburban - Since you are likely slowing for traffic, in gear is the choice.

City - In gear all the way.

Just my

 

Nope, not contradicting myself
I coast in gear until 1500 because there is rarely a situation where I can completely coast to a standstill in neutral.
If there was (plus no one behind me that I will piss off since I'm a considerate driver), I would.
Given that there isn't, I'm taking the hybrid approach to get the best of both worlds.

 

We can guess at which method is better but I'd realy like to know for sure because if I set out to save gas I want to do it right. I think you underestimate the fuel usage involved with keeping a car in nuetral. It's true that if you keep it in 6th for maximum distance you'll have to feed it some gas to get as far but I feel that the amount you'd have to give it is less than the amount that would have been burnt holding the engine in idle.

Someone mentioned puting it in nuetral and turning the car off entirely but that's unsafe and impractical.

 

Actually, I suspect you are over estimating the fuel usage at idle. I've seen numbers on American V-8's using .5 gallons per hour at idle. A TDI VW uses between .3 and .5 liters per hour at idle. I'd figure we use about a third of a gallon per hour at idle, seems reasonable, if not a little high.

We'll use the higher figure:

Given 0.5 gallons/hour
...we get 0.00834 gallons/minute
...assuming average in-gear slowdown down is 20 seconds of no fuel, we save 0.00278 gallons/incident
...in-gear idle down for 12 minutes to save 0.1 gallons,

You can see the savings is small...I'm not saying it's not worthwhile, after all I am now slowing in-gear rather than neutral when it makes sense.

My typical morning/evening commute would allow about 85 seconds/per or 2 minutes 50 seconds a day...so I now could save 55 gallons a year, but sometimes I choose the wrong strategy. Figure I'm saving about 20 gallons a year doing this, not bad for something you don't really have to think about after a while.

 

^ Understatement of the year IMHO.

 

Nope, the TSX will shut off the fuel injectors entirely in the latter situation (if you have taken your foot off the gas), so you literally burn no gas when engine braking...

 

I guess the difference is so small that we'll never realy know which is better, and the amount of gas saved is probably inconsequential, that having been being said and that having been said being said, I feel uncomfortable rolling in neutral because of inability to accelerate at a moments notice should it become necessary and the extra braking force required to slow the car, this being a car that has a shameful stopping distance to begin with.

 

Coasting is the way to go.

I got 500 miles out of one tank: https://acurazine.com/forums/showthread.php?t=43338

 

The big question is, did you coast in neutral or in gear?

 

I said the hypermileage hybrid guys do this (IIRC I saw it suggested on some forums), but I don't recommend it.

There are plenty of other ways - there are ten tips associated with this article.

 

Yea, rb1...again see post 38 in this thread for the Service Manual explaination.

 

When coasting to red lights and stop signs, I coast in gear(I will have to stop anyway and the difference in distance covered is usually minimal considering the visible distance on local streets), however when on slight downhills or approaching well anticipated stops I did coast in neutral to cover more distance.

Overall, I have noticed that there's very minimal difference between the two approaches with leaving the car in gear having a slight advantage over driving in neutral.

Of course, driving in neutral feels more comfortable(smooth) and requires less focus on the rpm and lock-out point and is generally prefered by drivers.

 

This explanation is referring to the safety cut-off and not fuel saving measures.

The fuel-cutoff in engine braking situations has been documented by monitoring the engine...

For example:

http://www.vtec.net/forums/one-messa...&page_number=1