North American English (NAE) flaps/taps and rhotic vowels have been shown to exhibit extreme variability that can be categorized into subphonemic variants. This variability provides known mechanical benefits in NAE speech production. However, we also know languages reuse gestures for maximum efficiency during speech production; this uniformity of behavior reduces gestural variability. Here we test two conflicting hypotheses: Under a uniformity hypothesis in which extreme variability is inherent to rhotic vowels only, that variability can still transfer to flaps/ taps and non-rhotic vowels due to adaptation across similar speech contexts. But because of the underlying reliance on extreme variability from rhotic vowels, this uniformity hypothesis does not predict extreme variability in flaps/taps within non-rhotic English dialects. Under a mechanical hypothesis in which extreme variability is inherent to all segments where it would provide mechanical advantage, including flaps/taps, such variability would appear across all English dialects with flaps/taps, affecting adjacent non-rhotic vowels through coarticulation whenever doing so would provide mechanical advantage. We test these two hypotheses by comparing speech-rate-varying NAE sequences with and without rhotic vowels to sequences from New Zealand English (NZE), which has flaps/taps, but no rhotic vowels at all. We find that NZE speakers all use similar tongue-tip motion patterns for flaps/taps across both slow and fast speech, unlike NAE speakers who sometimes use two different stable patterns, one for slow and another fast speech. Results show extreme variability is not inherent to flaps/taps across English dialects, supporting the uniformity hypothesis.