Friday, April 29, 2011
Coasting in Neutral or In Gear?
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 a Scan Gauge II
, one can 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
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!!!
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
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
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, the 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 a 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
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
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, the 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
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