To extend the molecular gas measurements to more typical star-forming galaxies ( SFGs ) with { SFR } < 40 ~ { } M _ { \sun } ~ { } yr ^ { -1 } and M _ { * } < 2.5 \times 10 ^ { 10 } ~ { } M _ { \sun } at z \sim 1.5 - 3 , we have observed CO emission with the IRAM Plateau de Bure Interferometer and 30 m telescope for five strongly-lensed galaxies , selected from the Herschel Lensing Survey . These observations are combined with a compilation of CO measurements from the literature . From this , we infer the CO luminosity correction factors r _ { 2 , 1 } = 0.81 \pm 0.20 and r _ { 3 , 1 } = 0.57 \pm 0.15 for the J = 2 and J = 3 CO transitions , respectively , valid for SFGs at z > 1 . The combined sample of CO-detected SFGs at z > 1 shows a large spread in star formation efficiency ( SFE ) with a dispersion of 0.33 dex , such that the SFE extend well beyond the low values of local spirals and overlap the distribution of z > 1 sub-mm galaxies . We find that the spread in SFE ( or equivalently in molecular gas depletion timescale ) is due to variations of several physical parameters , primarily the specific star formation rate , but also stellar mass and redshift . Correlations of the SFE with the offset from the main-sequence and the compactness of the starburst are less clear . The possible increase of the molecular gas depletion timescale with M _ { * } now revealed by low stellar mass SFGs at z > 1 and also observed at z = 0 is in contrast to the constant molecular gas depletion timescale generally admitted and refutes the linearity of the Kennicutt-Schmidt relation . A net rise of the molecular gas fraction ( f _ { gas } ) is observed from z \sim 0.2 to z \sim 1.2 , followed by a very mild increase toward higher redshifts , as found in earlier studies . At each redshift the molecular gas fraction shows a large dispersion , mainly due to the dependence of f _ { gas } on stellar mass , producing a gradient of increasing f _ { gas } with decreasing M _ { * } . We provide the first measurement of the molecular gas fraction of z > 1 SFGs at the low- M _ { * } end between 10 ^ { 9.4 } < M _ { * } / { M } _ { \sun } < 10 ^ { 9.9 } , reaching a mean \langle f _ { gas } \rangle = 0.69 \pm 0.18 which shows a clear f _ { gas } upturn at these lower stellar masses . Finally , we find evidence for a non-universal dust-to-gas ratio among high-redshift SFGs and sub-mm galaxies , local spirals and ultra-luminous infrared galaxies with near-solar metallicities , as inferred from a homogeneous analysis of their rest-frame 850 \mu m luminosity per unit gas mass . z > 1 SFGs show a trend for a lower L _ { \nu } ( 850 \mu { m } ) / M _ { gas } mean by 0.33 dex compared to the other galaxy populations .