There is reason to ask whether it is beneficial to decontaminate vehicles, in view of the great effort applied. If the level of contamination is low before the decontamination process, then the cost is not motivated, even if the decontamination is shown to be effective in relative terms. This report describes a trial made at the National NBC Defence School in Umeå and the calculations of contamination levels on vehicles and the resulting doses to the personnel. The radioactive isotope 24Na was dispersed along a 500 metres long loop of a gravel road and after that an All Terrain Carrier 206 was driven along the loop repeatedly. Samples were taken in 34 places on the vehicle after 1, 2, 3 and 6 kilometres and the amounts of 24Na in the samples were analysed. From the test results contamination and decontamination constants were calculated. The trial was made during winter conditions. The report ends with a description of the computer model for dose calculations which, together with the contamination and decontamination constants, was used for the calculation of doses to the personnel. The results show that the maximum contamination level on a ATC 206 is reached after just about 20 kilometres in a fallout area and that the self-decontamination is fast when the vehicle has been driven out of that area.