A series of rhombohedral alkaline-earth-doped manganites, La1−x (Ca,Sr)x MnO3, were studied, with x = 0.28 to 0.375, a constant tolerance factor of 0.927 ± 0.002, a very small variation of the Mn–O bond angle (<0.3°), and constant ion size variance (ς2) of 0.0020 ± 0.0010. Compositional uniformity and crystallinity were excellent, and porosity low. The temperature of the maximum in the resistivity, Tm, for x 4 0.28 was higher (by more than 20 K) than previously reported. With increasing x (and at the same time increasing Ca for Sr substitution), the Mn–O bond length shortened, Tm decreased, and the polaron hopping activation energy increased, indicative of increased carrier localization. The behavior is opposite to the La1−xSrxMnO3 series and can be explained by a strong dependence of the electron–phonon interaction on Mn–O bond length. In order to maximize Tm, Mn–O bond length must be controlled to a precise degree.