We present new interferometric observations for 74 luminous red stars, made in the near-infrared. We show that our 2.2 μm uniform-disk diameters agree with other near-infrared diameter determinations (lunar occultations and interferometers) for 22 stars measured in common with ours. From our new data, we derive effective temperatures that are compared with our previous work and with comparable observations made by lunar occultations at Kitt Peak. The combined data set yields 91 luminosity class II, II-III, and III stars that have well-determined spectral types spanning the range from about K0 to about M8. There are 83 stars in the sample that define an approximately linear relationship between spectral type and effective temperature for giants, with a dispersion of 192 K at each spectral type. Eight of the stars have temperatures that are roughly 750 K too low for their spectral types. These stars are not known to be at the high-luminosity end of the range of stars observed and are not recognized as binary stars. At present, we have no explanation for their low effective temperatures. We also show that Hipparcos parallaxes, combined with our angular diameters, yield linear radii precise enough to see differences in the average radius between luminosity class II and luminosity class III stars.