Free-molecular pressure probes are employed to deduce particle ﬂux in highly rareﬁed ﬂow ﬁelds, such as the farﬁeld or the backﬂow region of a plume expanding into vacuum. Analytical approaches to model the probe behavior are known in the literature for some time now. Numerical methods such as DSMC however allow to conduct investigations into the probe response in regimes inaccessible to analytical approaches. Besides, the numerical results may be compared to theoretical models in order to test their ﬁdelity. We computed the response of a probe with slot oriﬁce to a highly rareﬁed, parallel ﬂow with varying molecular speed ratio. We conﬁrmed the negligible inﬂuence of the speed ratio on probe response for low angles of attack, but observed signiﬁcant deviations from theory as the angle of attack approaches 90°.