The objective of the paper is to present the results of investigations to determine the effect of reduced tyre pressure on car lateral dynamics in an obstacle bypassing manoeuvre. This was done by performing the bench and road tests. The bench tests were carried out for parameterisation of the mathematical model of the tested car. The road tests covered the vehicle motion with reduced and no tyre pressure on a curvilinear track adequate for bypassing an unexpectedly appearing obstacle. Next, simulations in PC-Crash were performed, and the results were compared with those obtained in experiments.