Do you by chance mow with the PowerFlow Material collection system?
I have a '05 2305 (160hrs on it, also have a 3720 cab) and it sips gas (They both do),
seems like I can go 1/3 of the summer on a single fill. The one exception is when I ran (I don't use it anymore) the powerflow MCS.
The reason is that the powerflow is a blower so it's using a huge amount of HP all the time, whether or not you are sucking grass. (Actually, if you clog it it uses slightly less).
I easily doubled, if not tripled my fuel consumption when running with it. It was such a pain I run the mulching kit now.
I have a Craftsman 25hp (Kohler) GT5000 gasser (My girlfriend uses it at her place, I used to use it (pre 2005) at mine) and it uses much much more fuel than my 2305 for mowing the same area.