Introduction
This page discusses the advantages and disadvantages of using neutral, "N",
for coasting or a glide in a Prius.
It includes performance graphs generated from ODB, mini-scanner data
captured from an NHW11, 03 Prius.
Advantages
- Maximum glide
- Works with cold ICE
Disadvantages
- Moving violation when descending hills
- Risks overspeeding MG1 in transaxle, occurs when descending hill with ICE off
- Defeats auto-stop, ICE may continue to run at idle
Minimize Cold ICE Fuel Burn
When the ICE is cold, it has to run at least at idle to bring everything
up to operating temperature.
However, even with the accellerator totally off, there is a higher
fuel burn than when the ICE is at idle.
By putting the vehicle in "N" and coasting down a slope, a traffic moving
violation, one can start the journey at the maximum MPG:
I used this slope in the street I live on for transaxle oil rolling
resistance testing.
From a full stop, the car accellerated in neutral with some mechanical braking midway.
Regardless of speed, the ICE fuel consumption rate remained constant at idle.
This was half of the fuel usage rate needed to go 70 yards from my driveway.
"N" Glide, Cold ICE, Level Surface
In this test, the car started at 30 mph and I put it in "N" for maximum glide.
The ICE was still cold so it consumed fuel at the minimum, "idle" rate.
However, the coasting got me further on my trip and because this was a level
street, perfectly legal:
"N" Overpasses, Glide To Stop
In this test, speed was maintained via cruise control.
Earlier testing revealed that on the backside of an overpass, cruise control will often
overspeed and then go into ICE braking.
By going into "N" at the top of the overpass, the car maintains a steady speed on the
reverse slope, burning fuel at the idle rate, and avoiding the cruise control overspeed and
ICE braking effects.
Upon approaching the last overpass and signalling to get off, the car was put
in "N" to coast up and over the overpass.
Once on the access road, the car remained in "N" but even though it could turn
off the ICE, "N" defeats auto-idle and the ICE continues to run.
In this case, the ballistics glide was offset by the longer ICE burn because
"N" had defeated auto-shutdown.
Conclusions
Using "N" is effective and legal when the ICE is cold and on level streets to effect
a maximum efficiency glide.
Also, "N" will always achieve a maximum, ballistics glide compared to
manual accellerator techniques, there is no operator error.
However, "N" is a moving violation descending a hill; risks overspeeding MG1 if
descending a hill; and can defeat ICE autostop.