Originally Posted by
3dplane
It is a bit complicated. While it is true that during deceleration in gear above a certain rpm the fuel injectors are commanded off,so no fuel is injected. (Called DFCO or Deceleration Fuel Cut Off).
However it also slows down the vehicle (engine braking) thus killing momentum that could have been used to coast further.
Now if you were going to have to slow down anyway and would have to use the brakes to keep speed under control (like steep downhill),then sure it is a better option to leave the transmission in gear and let engine braking help staying off the brakes.
On the other hand if these are rolling hills and gaining too much speed on the downward leg is not a concern,I would neutral coast because the amount of fuel consumed at idle during the coast portion will be less than if I let engine braking hold me back from coasting further/faster and now I have to start accelerating sooner to make it up the next hill.
I don't know if that made sense,but it really boils down to the particular situation/terrain conditions.
100% That's what I do now when we go to the city. Have to climb a good size mountain, and on the way home coasting would get me doing 160 or so in a 100 Leaving it in 5th usually lets it creep up to 120 with some minimal braking.
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View my fuel log 2014 Mirage SE 1.2 manual: 52.3 mpg (US) ... 22.2 km/L ... 4.5 L/100 km ... 62.8 mpg (Imp)