Top: iron abundances of Galactic classical Cepheids vs. Galactocentric distance. Spectroscopic measurements based on different data sets are plotted with different colors. They include updated iron abundances based on current UVES spectra (30, red) and on multi-epoch UVES spectra (11, dark red); as well as updated iron abundance for CE Pup based on a FEROS spectrum (light blue) and iron abundances provided by our group: Genovali et al. (2013) (33, blue), Lemasle et al. (2007), Lemasle et al. (2008) (39, yellow), Romaniello et al. (2008) (14, green). Cepheids that according to the General Catalog of Variable Stars (Samus et al. 2009) are candidate classical Cepheids were plotted with open circles. The gray dashed line shows the metallicity gradient. Middle: same as the top, but the iron abundances include our measurements and those available in the literature: Luck et al. (2011), Luck & Lambert (2011), and Sziládi et al. (2007) (322, black). The gray dashed line shows the metallicity gradient based on the entire sample. Bottom: zoom of the top panel for Galactocentric distances ranging from 4 kpc to 20 kpc. The bars on individual Cepheids display the uncertainty both on iron abundance and on distance. The vertical black bar in the bottom right corner shows the mean uncertainty on Luck & Lambert (2011) and Romaniello et al. (2008) iron abundances.