I have an L3000DT and I thought the fuel gauge was not working when I first got my tractor. When it is full to the top it takes a while for the gauge to move off the full mark.
Once the gauge gets pass the 3/4 full mark it decends at a much more rapid rate. As far as hours per gallon it is really hard to say, but I'd guess it's close to 1 1/4 to 1 1/2 hours per gallon for use a little under PTO rpm speed.
I don't think I have yet to burn through a half of tank of fuel in what I consider a full days work, but then I don't do any mowing or brush hog work.
Randy