My L3400 seems to like burning the fuel. Compared to my old Case IH 1140 that was about 10 less hp. I have spent some time on my uncles B2920 mowing with 5ft finish mower and I will say that my L3400 burns way more fuel mowing with my 6ft finish mower. Both machines are HST. More power equals more fuel is the way it goes.
Our L3600GST seems to burn just a touch more that 1.25 gal/hour when running a 84" Befco finish mower. After mowing for 3 hours, 3.75-4 gals tops off the tank vertually every time (thats with running the A/C). Even the little BX2230 will burn just more than 1 gal/hour mowing with the 60"mmm.
More power doesn't always mean that you will burn more fuel. For instance our F350 4X4 7.3L, when it was bone stock mileage running empty was 16-16.5, now with lots of modifications and 425hp/900+lbft to the rear wheels the truck pulls down 19-20 mpg if your nice on the throttle. This is because the motor doesn't have to work nearly as hard to do the same amount of work.
I haven't been able to measure my fuel usage with any exactness yet, but my tractor was delivered with "about" half a tank and ran over 8 hours of mowing before I added any fuel. I am only at 13 hours now and have not used the next 5 gals yet.
Mine is about the same mowing moderate fields with the rotary cutter. I get a bit better doing loader work, but that's a safe number for the 3400. I doubt you'd use much more than that unless you had some serious load all the time.
Three hours of finish mowing (Woods RD7200) takes about 3.5 gallons on my L3400. That's mowing my big yard which I let get to about 5 or 6 inches before I mow. That include backing up a lot for about 40 trees.