the like as the breather pipe is thus consealed by the seat or the like the fuel tank assembly of fig1 presents a neat appearance the assembly is simple in construction and inexpensive a fuel tank assembly according to a third embodiment of this invention is fragmentarily shown in fig1 and 12 and a modification thereof appears in fig1 this embodiment is characterized by the location of the breather pipe 26 within the fuel pipe 25 the breather pipe 26 may be concentric with the fuel pipe 25 as shown in fig1 and 12 or eccentric as shown in fig1 this arrangement facilitates the piping work between the main and auxiliary tanks and the fuel tank assembly presents a neat appearance a fourth embodiment of this invention is shown in fig1 and 15 and is characterized by a breather pipe 26 lying on and secured to the bottom of a main tank 7 the pipe 26 is secured more firmly than according to the first embodiment in which the breather pipe is welded to the bottom plate of the main tank only at a point where the pipe extends through the bottom plate the pipe 26 has a portion 26d lying along and secured to the bottom plate 7b of the main tank 7 and an upwardly bent end portion 26e opening in the vicinity of a fuel inlet 7a.