Articulatory synthesis relies on precise, parametric vocal tract shapes to generate natural-sounding speech. In German, a particular challenge is the accurate synthesis of the vocalic /r/ allophones following vowels or in syllable coda position. Using established phonetic conventions, no satisfying results could be achieved so far, implying a possible shortcoming of these existing conventions. This study therefore analyzed a large number of natural recordings of the sounds in question from a single speaker to find the optimal number of target vocal tract shapes. Applying clustering techniques, the manifold of vocalic /r/ allophones could be reduced to two prototypical [] variants previously undescribed in the literature. As shown by a listening experiment, which of these two sounds was preferred in which context depended not only on the respective context vowel's tenseness, but also on its openness and acoustic distance to other context vowels ending in the same [] variant. This indicates that the two different allophones might serve as a contrastive cue to help differentiate between otherwise similar context vowels.