Fuel capacity vs reserve in motorcycles?

The fuel petcock has two tubes that extend up into the fuel tank. The longer one is for the "on" position, and the shorter one is for the "reserve" position. When the tank gets to the top of the on position, you "run out of gas" and can switch to the reserve position, using the lower tube, which will give you a few more miles, depending on how different the levels are on the two tubes and the shape of the tank at that fuel level. This is not a substitute for watching the odometer, and may still leave you stranded if you use it as such.