Accuracy of WIM (Weigh-In-Motion) systems depends on several factors, of particular importance is the calibration procedure. Parameters of the calibration process have essential influence on accuracy of weighing results. There are many calibration methods to be used for weighing systems. The pre-weighed calibration vehicles method is the most commonly applied, because it is simple and applicable to various classes of WIM systems. There are a few estimators of calibration coefficients possible to determine, according to the assumed criterion. This paper summarizes the results of estimator quality assessment, for a number of trucks and their passing cycles. Besides, the paper is intended to explore how parameters of the calibration process, the quality of the road surface, class and speed of calibration vehicles could influence the final accuracy of WIM systems. Such analysis requires a number of different tests in different conditions and is rather time consuming, that is why modelling and simulation methods were adopted instead.