We report new interferometric angular diameter observations of 15 carbon stars. Combining these with previously published diameters for seven other carbon stars, we are able to compute effective temperatures for 22 stars. Of these, 16 have Yamashita spectral classes and are of sufficiently high quality that we may determine the dependence of temperature on spectral type. We find that (with three exceptions) there is a small dispersion of effective temperatures, in the range of spectral classes C5-C9, with a mean value of 3000±200 K (rms). There is a slight tendency for the effective temperature to increase with increasing temperature index, in agreement with a previous finding of Tsuji. We assess the effects of circumstellar shells upon our temperature determinations and find that only S Aur and CIT 13 are likely to be contaminated by the presence of a shell. The mean radius for the carbon star sample is estimated to be about 400 Rsun, which makes these stars more similar (in effective temperature and radius) to Mira variables than to late M giants.