Comparisons of observations and models of SiO (8−7) to (3−2) integrated intensity ratios, extracted over the red-shifted wing of the observed line profiles. For observations, the ratios are displayed against the width of the velocity range over which they are extracted. For models, the ratios are plotted as a function of the shock velocity of the model. In the context of 1D shock modelling, the width of the velocity range used for observations should be directly comparable with the shock velocity of the models for each observed outflow. The observations are shown in coloured squares for various objects in our sample. The empty circles are the models of Gusdorf et al. (2008a) for pre-shock densities of 10⁴, 10⁵, 10⁶ cm^-3, the filled diamonds are the models (n_H = 10⁵ cm^-3) of Anderl et al. (2013) without SiO initially in the grain mantles, and the filled flat hexagons are the same, including an initial amount of SiO in the grain mantles.