[jbarberu]

When you go in neutral downhill your engine idles, vs if you leave it in gear the ECU cuts fuel supply completely. It's generally better to stay in gear except when you're at a standstill. (This assumes a modern vehicle with fuel-injectors)

[marvin]

Do automatic gearboxes compression brake when going downhill? That's what I was trying to avoid. My point was to conserve your kinetic energy, not optimizing the fuel flow during the times when you don't want the engine to produce power.

[jbarberu]

Can't speak for all of them, but many will engine brake if you use cruise control downhill.