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Characterizing accurate vs. misarticulated patterns of tongue movement using ultrasound can be challenging in real time because of the fast, independent movement of tongue regions. The usefulness of ultrasound for biofeedback speech therapy is limited because speakers must mentally track and compare differences between their tongue movement and available models. It is desirable to automate this interpretive task using a single parameter representing deviation from known accurate tongue movements. In this study, displacements recorded automatically by ultrasound image tracking were transformed into a single biofeedback parameter (time-dependent difference between blade and dorsum displacements). Receiver operating characteristic (ROC) curve analysis was used to evaluate this parameter as a predictor of production accuracy over a range of different vowel contexts with initial and final /r/ in American English. Areas under ROC curves were 0.8 or above, indicating that this simple parameter may provide useful real-time biofeedback on /r/ accuracy within a range of rhotic contexts.