This study examined effects of phonemic vowel length and speaking rate, two factors that affect vowel duration, on the first and second formants of all vowels in Japanese. The aim was to delineate the aspects of formant displacement that are governed by the physiological proclivity of vowel production shared across languages, and the aspects that reveal language-specific phenomena. Acoustic analysis revealed that the phonemic long vowels occupied a more peripheral portion of the F1 × F2 vowel space than the phonemic short vowels (effect of vowel length), but effects of speaking rate were less clear. This was because of the significant interactions of the two effects: the formants of phonemic short vowels were more affected by speaking rates than the phonemic long vowels. Regression analyses between F2 and duration revealed that formant displacement occurs when vowels are less than 200 ms. Similarities and differences found for Japanese and English are discussed in terms of physiological proclivity of vowel production versus language-specific phonological encoding.