also depends on the load etc. My BX uses that much when using the BH at 95% throttle. Mowing is less the BH...for me.
But more stuff you have running cost more. 5-gallons could be right if you have an older tractor, etc.
Maybe get some other input. I know my BX does not use much and my neightbor has an older Ford 1220 and his uses about the same.
As posted above if it not smoking (black) or have zero fuel in oil, it just burns a lot of fuel.