"Tank should not be run below approx. 1/8-1/4 due to sloshing in the unbaffled tank."
From the Vulcan site. I have run my tank really really low too. Its just not a good idea

dam i get 19 gallons and that runs my truck out of fuel its deffinly not getting all it should haha i just put a new in tank pump unit and now my gauge is messed up. i filled it and calculated the miles 16.7mpg ablout 200miles and it gets really close to running out got too do somthing just dont want to drop the tank again