To investigate the mechanical strength of double network (DN) gels in a wide dynamical range, the gel solvent was replaced with viscous solvents-ethylene glycol and glycerol-and with their mixtures with water. The viscosity dependence of fracture energy in a tearing test was weak but demonstrated the existence of characteristic tear velocity for maximum fracture energy as a function of solvent viscosity. Fast and slow relaxation modes in the gels' dynamics were simultaneously observed by scanning microscopic light scattering, which reveals that the DN-gel structure stays unperturbed by the solvent change. These results implied that the solvent viscosity weakly affect the toughness and structure of the DN gels. Further, we demonstrated that the effect of solvent viscosity on the fracture energy can be incorporated into a scaled tear velocity that distributes over 6 orders of magnitude. We concluded that the viscous friction between polymer and solvent is not particularly significant for the energy dissipation of DN-gel fracture.