Left: TIR luminosity (8 − 1000 μm), tracing the star formation rate, as a function of HCN luminosity, tracing dense molecular gas. Right: TIR/HCN, tracing the dense gas star formation efficiency, as a function of HCN luminosity. In both panels, we compare against measurements: (a) for whole galaxies (Gao & Solomon 2004a,b; Graciá-Carpio et al. 2008; Krips et al. 2008; Juneau et al. 2009; Crocker et al. 2012; García-Burillo et al. 2012; Privon et al. 2015); (b) kpc-size parts of galaxies (Kepley et al. 2014; Usero et al. 2015; Bigiel et al. 2015, 2016; Gallagher et al. 2018a); (c) resolved clouds and cores (Chin et al. 1997, 1998; Brouillet et al. 2005; Wu et al. 2010; Buchbender et al. 2013; Stephens et al. 2016; Braine et al. 2017). Our new datapoints in M 51 at 3″ resolution (∼100 pc) are displayed as filled circles, while the gray circles show similar-scale measurements in the outer spiral arm of M 51 from Chen et al. (2017).