I have a 6.6 gallon tank. I've been told that with the lines and filter, and all, there is about 6.5 gallons of usable fuel. I miscalculated today, too much head wind, wet pavement, and the heavy trailer took it's toll. I slowed down to 45 mph for about 10 miles, made it to Vale, OR. It took 6.48 gallons! Came out to 36 something mpg, I'm used to around 41 with a trailer.