Some catastrophic ﬁres occurred in the last 15 years in both road and railway tunnels usedby millions of people are turning the attention of the engineering community towards the assessmentof the structural safety of tunnels, both during and after a ﬁre. To this end, a realistic description ofﬁre propagation and a reliable picture of the thermal ﬁeld are mandatory, in order to evaluate thethermal load acting on the lining and the partitions. With reference to a rather common road-tunnelgeometry, three possible thermal inputs representing as many levels of ﬁre severity are consideredhere, namely the burning of a car, of a bus and of a heavy-goods vehicle (HGV). The ﬁre scenariois ﬁrstly modeled by means of a rather simple but handy code based on a two-zone model for thedescription of the ﬁre in each compartment. Later, a more complex code based on computational ﬂuiddynamics applied to the simulation of ﬁre development (Fire Dynamics Simulator - FDS) is used toallow a comparison between the numerical results obtained in either way, and to check to what extentsome available experimental results concerning similar tunnels can be ﬁtted. The ﬁtting shows that- by properly subdividing the tunnel (that has no compartments) into a suitable number of “virtual”compartments - the agreement between the two codes is very good, as well as that between numericaland experimental results.