Coasting in Neutral or In Gear?

I found plenty of opinions and bad science on this topic but there is no conclusive evidence available to settle this debate.  Which one saves more gas when coasting?  Shifting into neutral or staying in gear?

My philosophy is:  If you can't find someone from which to copy the answer, FIGURE IT OUT YOURSELF! Then write a blog about it.

The Neutral Camp: Shifting into neutral allows the engine to run at its lowest RPM but the engine has to burn a small amount of fuel to keep itself running. Very little resistance is applied to the motion of the vehicle.

The Keep it in Gear Camp: Keeping it in gear, the vehicle's computer senses the reduction in power demand and shuts off fuel to the fuel injectors. When costing in gear, the engine is essentially off. This added resistance acts like an engine break, slowing the vehicle down but it does so with almost no fuel consumption.

Both sides have a valid argument. As I research ways to achieve higher fuel efficiency, I find myself changing sides back and forth. 

Using my newly acquired Scan Gauge II, I am now able to accurately measure instantaneous fuel economy. Since the late 1980's, every single fuel injected vehicle on the planet has had the ability to monitor its instantaneous fuel economy but almost every automobile manufacturer chooses not to include this "planet saving" feature in their vehicles. Why? I don't know.

While driving my wife's 2002 Chevy Venture, I found a less-busy section of interstate where I could setup and run the test.  With the aide of my mom who was traveling with me, I recruited her to assist me in recording the data. 47 years ago, she typed my dad's Masters Thesis.  Mom was more than capable of writing down a few readings while I focused on keeping the van traveling between the lines.

After I accelerated above the desired speed, I let off the gas until the van began coasting. This allowed the van to settle out and decelerate to the correct speed. Note:  All freeway speed measurements were taken on a flat, strait stretch of road during a high wind storm. This unusual weather provided a nearly constant 25mph tail-wind which allowed the vehicle to decelerate more slowly. This aided me in taking more accurate, high speed readings.

Initially, the data was inconclusive. The first 2 sets of data showed that coasting in neutral got better gas mileage. The second set of data points revealed that keeping the van in gear was the more fuel efficient choice.

I decided to try this experiment again later that night, this time running at slower speeds in an undeveloped residential neighborhood. By this time, the wind had died down to only a few mph.  With the aid of my 9 year old daughter, we ran 2 sets of “coast tests” at 30mph and 2 sets at 25mph. Each set was driven in an opposite direction to eliminate the wind as a factor. But once again, the first 2 tests revealed that Neutral is the superior choice and 2nd set of tests demonstrated that coasting in gear was better.  What is going on here?

Glancing down at the Scan Gauge II display, it hit me. Engine temperature!!!

When an engine first starts up, it is running at temperatures that are out of the operating range of some of its sensors. To combat this, the vehicle's control system runs in an "open loop" (does not receive feedback from its sensors) so it plugs in default values for the sensors.
As the engine warms up, the engine's control system switches over to “closed loop” where it makes decisions based on the actual readings from the sensors.

While running with a cold engine, coasting in gear gets lower fuel efficiency and coasting in neutral achieves higher efficiency.  After the engine warms up, the opposite is true. 
Update 5/1/2011:  The effect I was seeing is not related to the closed loop/open loop.  After more tests, my wife's Chevy Venture goes into closed loop within a few seconds of starting up the engine.  Whatever the phenomenon, the increased in-gear efficiency only happens after about 10 minutes of driving and after the engine coolant temperature reaches about 185 F.  I assume it is when the thermostat opens.  Whatever the effect, it is re-producible, predictable and measurable.

The following readings were taken on 3 separate occasions, each while the the engine was cold.

Note:  instantaneous fuel efficiency readings can be extremely high while coasting at high speed. 

Engine very cold in the early morning
75mph run   Gear       mpg
Run#0          Neutral    140mpg
Run#0          In Gear     97mpg

Engine cold in afternoon and very high tail-wind
65mph run   Gear         mpg         RPM
Run#1          Neutral      176mpg     900rpm
Run#1          In Gear      170mpg    1800rpm
Run#2          Neutral      188mpg     914rpm
Run#2          In Gear      186mpg    1630rpm

Engine cold at night and low/no wind
30mph run   Gear        mpg          RPM
Run#3          Neutral      82mpg       769rpm
Run#3          In Gear      69mpg       894rpm
Run#4          Neutral      89mpg       737rpm
Run#4          In Gear      76mpg       878rpm

The following readings were taken on 2 separate occasions, each after the engine had warmed up.

Engine warm ~195 degrees
25mph run   Gear        mpg           RPM
Run#1          Neutral      62mpg        858rpm
Run#1          In Gear      64mpg        875rpm
Run#2          Neutral      63mpg        858rpm
Run#2          In Gear      64mpg        855rpm
Run#3          Neutral      67mpg        814rpm
Run#3          In Gear      69mpg        828rpm

30mph run   Gear        mpg            RPM
Run#4          Neutral      80mpg        858rpm
Run#4          In Gear      83mpg        847rpm
Run#5          Neutral      82mpg        825rpm
Run#5          In Gear      85mpg        792rpm

55mph run  Gear          mpg           RPM
Run#6         Neutral       154mpg       890rpm
Run#6         In Gear       155mpg      1080rpm

70mph run  Gear         mpg           RPM
Run#7         Neutral       200mpg      900rpm
Run#7         In Gear       205mpg      2000rpm (WOW!!)

Conclusion: With a cold engine, coasting in neutral uses 12% less fuel than coasting in gear. (at least for the default values in the computer of my wife's 2002 Chevy Venture). When the engine is warmed up, it flips the other way and coasting in gear saves on average 2.7% more fuel than coasting in neutral.

If you drive a manual transmission and have a short commute (less than 15 minutes), you will be better off coasting in neutral as most of your commute will take place with a cold engine. If you drive an automatic and/or have a commute that lasts longer than 15 minutes, coast in gear. It's probably better to leave it in gear anyway just for the safety and convenience aspects.

I may only be getting only 20mpg while driving 75mph but during that 5 seconds span when I am coasting at freeway speeds before pushing on the gas again, my van is getting over 200mpg.
Using a tool like the Scan Gauge II allows for super accurate measurements of high fuel economy during these very specific types of driving.
Although these short periods of excellent fuel economy make up only a small portion of a normal commute, they contribute to the overall fuel and cost savings.
Now that each one has been quantified, you can now decide which method best suites your personal driving preferences. For a warm engine, there is only 2.5% of variation between the two methods and for most people, that's small enough to say “who cares?” That's fine but at least now we know.