A transport refrigeration system for a transportable temperature controlled space includes a compressor, a condenser and an evaporator fluidly connected to one another, an engine operatively connected to the compressor, and a fuel tank assembly fluidly connected to the engine. The fuel tank assembly includes a fuel tank configured to contain the fuel, a fuel vapor and air, and an ultrasonic fluid level sensor for sensing a level of fuel, the fuel tank having a maximum fuel level. The fluid level sensor includes a transducer having a ring period. The distance from the fluid level sensor to the maximum fuel level is at least half the distance that the sound travels through the fuel vapor and air during the ring period of the transducer.