A vehicle fuel tank assembly is provided with a fuel tank and a fuel pump unit. The fuel tank is mounted on a saddle-riding type vehicle. The fuel tank has a flat shape elongated in the front-to-rear direction. The fuel pump unit includes a main body, a support body extending from the upper part of the main body in the longitudinal direction of the fuel tank, and a fuel level gauge supported and extending on the support body. The fuel level gauge includes a fuel level gauge body, a float arm pivotally supported by the gauge body, and a float on the free end of the float arm. The float arm extends in the longitudinal direction of the fuel tank and takes a highest swing position in an intermediate level of the height of the fuel tank. The fuel tank assembly enables fuel level detection in a wide range.