Full, even when I had my 22 gallon Montana's tank to fill. It only takes a few minutes. By filling only half way you're raising the chance of enough water getting into your tank via condensation that it may impact the car. A full tank will mix any condensation in and keep it diluted to a level that the car doesn't notice it.

 

Since I live in NY and work in NJ where gas is cheaper I fill back up at night on my way home.

 

I fill all my tanks full except when I will be going to the track, lighter is faster.

 

i fill every thursday , even if I've only driven it once, keeps the fuel new and the engine clean

 

Filling full makes it easier to use fuelly also. I typically won't worry about it until under half anyway unless gas prices are cheap. That ain't going to happen so.......... why I was happy to get an ECO instead of my LS!