The results and findings of three previous research projects are combined with new research to estimate the overall influence of longitudinal ventilation on fire size and spread in tunnels. Each of the three previous projects is briefly described. Combining the results of these three projects, together with knowledge of HGV fire behaviour in an experimental test, enables the estimation of the maximum fire size of a fire in a tunnel and the conditions under which it might spread to an adjacent vehicle, for a given longitudinal ventilation velocity. These results have been combined into a single computer model. Results are presented and discussed. It is concluded that, although it may greatly increase the heat release rate of a fire in a tunnel, increasing the ventilation velocity will tend to reduce the likelihood of the fire spreading to an adjacent vehicle, assuming no flame impingement.