We present a new virial equation of state for natural gas systems under 'custody transfer conditions', defined at 270 less than or equal to T (K) less than or equal to 330 and P less than or equal to 12 MPa. This model predicts compression factors of natural gases with at least the same accuracy as the well-known GERG virial equation of state. However, the model is devised in such a way that the prediction of caloric properties, particularly the speed of sound, is significantly improved with respect to the GERG virial equation. This was achieved by representing the second and third virial coefficients as quadratic functions of inverse temperature. When the new model was compared with a large database of experimental results, the overall absolute average deviation in compression factors was 0.034% while that in speeds of sound was 0.17%.