fuel consumption ponder

[vprelovac]

I've been wondering what mode of driving keeps the consumption down more.

Lets say you are descending down the smooth hill in your fifth gear.

option a) you leave the gear in fifth and depress the gas pedal entirely
option b) you switch to idle

in first case the consumption measured by board computer is 0.0 although revs are up. in second mode, the engine is idle but the board comp shows some value (2-3 liters/100km). so what is the truth there? i've been thinking the whole time that the idle-going is best way to save fuel but my s2 comp shows me otherwise. help solve my dilemma !!

[LHD_Kid]

This was tried on BBC televisions Top Gear a few years ago, with an Audi A8...
Best way IS in 5th gear, as gravity helps to turn the engine, thereby using less fuel, on tick over it is only the fuel running it. Same applies every time you stop, leave in gear till last minute, etc. heater off (especially A/C ofcourse...!), windows closed, even radio off, etc... all have an effect

[andrewd]

A lot of cars actually cut fuel off completely on the overrun. (I'm reliably informed that the urS4 engine management does this - not sure about the S2!) So ...dipping the clutch and rolling along means that a small amount of fuel needs to burned to keep the engine running, but completely lifting off the gas and letting it coast in gear uses no fuel at all!

[hiltoa]

I was always told to use engine braking - it's free! I think pretty much all petrol cars since at least the mid 90's cut the fuel on overrun as part of the drive ( ) to decrease emissions.

[vprelovac]

Hmm that pretty much sums it up I guess.

Another question - no matter how hard I press gas pedal the maximum consumption never exceeds 51-52 liters. Is this limited by injector size or by map in the computer? I mean if for example bigger injectors are put would it use more? And would there be more power on maximum gas? Sorry if the questions asked are somewhat noobish